feat(tinytorch): make the website an installable PWA

Add web app manifest, service worker, and app icons so the TinyTorch
site can be installed to the home screen and read offline.

- manifest.webmanifest with relative start_url/scope so it works both
  at mlsysbook.ai/tinytorch/ and in local previews; any + maskable
  icons generated from the flame mark in logo-tinytorch-simple.png
- sw.js: network-first for pages (never stale after a deploy, cached
  fallback offline, branded offline page as last resort),
  stale-while-revalidate for same-origin css/js/images/fonts,
  passthrough for JSON/data so release-manifest and search stay live
- assets/scripts/pwa.html registers the worker at the site base path,
  derived at runtime since pages render at several directory depths
- fix apple-touch-icon to a real 180x180 icon (the old href pointed at
  the 583x328 wordmark)

Author: farhan523

tinytorch/quarto/_quarto.yml

@@ -20,10 +20,18 @@ project:
   # install.sh and community/ moved here from the retired tinytorch/site/
   # (Jupyter Book) tree so this quarto tree is the only source for
   # anything published under /tinytorch/.
+  #   - manifest.webmanifest / sw.js / assets/images/icons/**
+  #                          PWA installability + offline support. The
+  #                          manifest link and service worker registration
+  #                          are injected by assets/scripts/pwa.html, so
+  #                          Quarto never sees them referenced statically.
   resources:
     - assets/downloads/**
     - install.sh
     - community/**
+    - manifest.webmanifest
+    - sw.js
+    - assets/images/icons/**
 
 website:
   title: "TinyTorch"
@@ -269,14 +277,19 @@ format:
     include-in-header:
       - file: ../../shared/config/site-head.html
       - text: |
-          <link rel="apple-touch-icon" href="/assets/images/logos/logo-tinytorch.png">
+          <!-- Leading-/ hrefs are project-root-relative: Quarto rewrites
+               them per page depth (../ on nested pages), so they resolve
+               correctly under mlsysbook.ai/tinytorch/ and local previews. -->
+          <link rel="apple-touch-icon" sizes="180x180" href="/assets/images/icons/apple-touch-icon.png">
+          <link rel="manifest" href="/manifest.webmanifest">
           <meta name="theme-color" content="#D4740C">
           <!-- Release identity (rendered into the footer pill below).
                The publish workflow drops release-manifest.json at
                /tinytorch/release-manifest.json before deploy; preview
                and local renders silently no-op. -->
           <meta name="release-manifest" content="/tinytorch/release-manifest.json">
     include-after-body:
+      - file: assets/scripts/pwa.html
       - file: assets/scripts/subscribe-modal.js
       - file: assets/scripts/ml-timeline.js
       - file: assets/scripts/sidebar-subtitle.html

tinytorch/quarto/assets/images/icons/apple-touch-icon.png

@@ -0,0 +1,28 @@
+<script>
+/*
+ * PWA bootstrap — registers the service worker (sw.js at the site root, so
+ * its scope covers the whole TinyTorch site). The manifest link lives in
+ * include-in-header (_quarto.yml); only the worker needs JS because
+ * registration takes a URL resolved against the page, and pages render at
+ * several directory depths. The site deploys under /tinytorch/ on
+ * mlsysbook.ai but local previews serve the build at the server root, so
+ * the base path is derived at runtime to handle both.
+ */
+(function () {
+  'use strict';
+  var secureContext = window.location.protocol === 'https:' ||
+      window.location.hostname === 'localhost' ||
+      window.location.hostname === '127.0.0.1';
+  if (!('serviceWorker' in navigator) || !secureContext) return;
+
+  var match = window.location.pathname.match(/^(.*\/tinytorch\/)/);
+  var base = match ? match[1] : '/';
+  window.addEventListener('load', function () {
+    navigator.serviceWorker.register(base + 'sw.js').catch(function (err) {
+      // Expected on previews that serve the build under a different root;
+      // the site works identically without the worker.
+      console.warn('TinyTorch PWA: service worker registration failed:', err);
+    });
+  });
+})();
+</script>

tinytorch/quarto/assets/images/icons/icon-192.png

@@ -0,0 +1,38 @@
+{
+  "name": "TinyTorch — Build Your Own ML Framework",
+  "short_name": "TinyTorch",
+  "description": "An interactive course for building machine learning systems from the ground up. Learn by implementing your own PyTorch-style framework.",
+  "id": "./",
+  "start_url": "./",
+  "scope": "./",
+  "display": "standalone",
+  "orientation": "any",
+  "background_color": "#ffffff",
+  "theme_color": "#D4740C",
+  "icons": [
+    {
+      "src": "assets/images/icons/icon-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "assets/images/icons/icon-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "any"
+    },
+    {
+      "src": "assets/images/icons/icon-maskable-192.png",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "maskable"
+    },
+    {
+      "src": "assets/images/icons/icon-maskable-512.png",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ]
+}

tinytorch/quarto/assets/images/icons/icon-512.png

@@ -0,0 +1,155 @@
+/*
+ * TinyTorch service worker — offline support for the PWA.
+ *
+ * Strategy:
+ *   - Navigations (HTML): network-first. The site deploys frequently, so
+ *     pages must never go stale; the cache is only a fallback for offline
+ *     revisits, with a branded offline page as the last resort.
+ *   - Same-origin static assets (css/js/images/fonts): stale-while-revalidate.
+ *     One deploy of staleness is acceptable for assets and keeps repeat
+ *     visits fast.
+ *   - Cross-origin requests (Google Fonts, cdnjs) are left to the browser —
+ *     caching opaque responses inflates storage quota unpredictably.
+ *
+ * Bump CACHE_VERSION when changing caching behavior; old caches are dropped
+ * on activate.
+ */
+
+'use strict';
+
+const CACHE_VERSION = 'v1';
+const PAGE_CACHE = `tinytorch-pages-${CACHE_VERSION}`;
+const ASSET_CACHE = `tinytorch-assets-${CACHE_VERSION}`;
+const ASSET_CACHE_LIMIT = 200;
+
+// Everything the offline fallback needs is inlined below, so install only
+// warms the cache with the manifest and install-prompt icons.
+const PRECACHE_URLS = [
+  './manifest.webmanifest',
+  './assets/images/icons/icon-192.png',
+  './assets/images/icons/icon-512.png',
+];
+
+const OFFLINE_HTML = `<!DOCTYPE html>
+<html lang="en">
+<head>
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<title>Offline — TinyTorch</title>
+<style>
+  body {
+    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
+    display: flex; align-items: center; justify-content: center;
+    min-height: 100vh; margin: 0; background: #fff; color: #333;
+    text-align: center; padding: 1rem;
+  }
+  @media (prefers-color-scheme: dark) {
+    body { background: #1a1a1a; color: #ddd; }
+  }
+  .flame { font-size: 4rem; }
+  h1 { margin: 0.5rem 0; font-size: 1.5rem; }
+  p { color: #888; max-width: 28rem; }
+  button {
+    margin-top: 1rem; padding: 0.5rem 1.5rem; font-size: 1rem;
+    background: #D4740C; color: #fff; border: none; border-radius: 6px;
+    cursor: pointer;
+  }
+</style>
+</head>
+<body>
+<main>
+  <div class="flame">🔥</div>
+  <h1>You're offline</h1>
+  <p>This TinyTorch page isn't cached yet. Pages you've visited before are
+  available offline — reconnect to load this one.</p>
+  <button onclick="location.reload()">Try again</button>
+</main>
+</body>
+</html>`;
+
+self.addEventListener('install', (event) => {
+  event.waitUntil(
+    caches
+      .open(ASSET_CACHE)
+      .then((cache) => cache.addAll(PRECACHE_URLS))
+      .then(() => self.skipWaiting())
+  );
+});
+
+self.addEventListener('activate', (event) => {
+  event.waitUntil(
+    (async () => {
+      const names = await caches.keys();
+      await Promise.all(
+        names
+          .filter((n) => n.startsWith('tinytorch-') && n !== PAGE_CACHE && n !== ASSET_CACHE)
+          .map((n) => caches.delete(n))
+      );
+      if (self.registration.navigationPreload) {
+        await self.registration.navigationPreload.enable();
+      }
+      await self.clients.claim();
+    })()
+  );
+});
+
+async function trimCache(cacheName, limit) {
+  const cache = await caches.open(cacheName);
+  const keys = await cache.keys();
+  if (keys.length <= limit) return;
+  // Cache keys are ordered oldest-first; drop from the front.
+  await Promise.all(keys.slice(0, keys.length - limit).map((k) => cache.delete(k)));
+}
+
+async function networkFirstPage(event) {
+  const cache = await caches.open(PAGE_CACHE);
+  try {
+    const response = (await event.preloadResponse) || (await fetch(event.request));
+    if (response && response.ok) {
+      cache.put(event.request, response.clone());
+    }
+    return response;
+  } catch (err) {
+    const cached = await cache.match(event.request);
+    if (cached) return cached;
+    return new Response(OFFLINE_HTML, {
+      status: 503,
+      headers: { 'Content-Type': 'text/html; charset=utf-8' },
+    });
+  }
+}
+
+async function staleWhileRevalidate(request) {
+  const cache = await caches.open(ASSET_CACHE);
+  const cached = await cache.match(request);
+  const network = fetch(request)
+    .then((response) => {
+      if (response && response.ok) {
+        cache.put(request, response.clone());
+        trimCache(ASSET_CACHE, ASSET_CACHE_LIMIT);
+      }
+      return response;
+    })
+    .catch(() => undefined);
+  return cached || network.then((r) => r || Response.error());
+}
+
+self.addEventListener('fetch', (event) => {
+  const request = event.request;
+  if (request.method !== 'GET') return;
+
+  const url = new URL(request.url);
+  if (url.origin !== self.location.origin) return;
+
+  if (request.mode === 'navigate') {
+    event.respondWith(networkFirstPage(event));
+    return;
+  }
+
+  const dest = request.destination;
+  if (dest === 'style' || dest === 'script' || dest === 'image' || dest === 'font') {
+    event.respondWith(staleWhileRevalidate(request));
+  }
+  // Everything else (search.json, release-manifest.json, XHR/fetch data)
+  // goes straight to the network so dynamic data is never served stale.
+});