docs(webapp): document the Query devtools flag in the README

The opt-in was only discoverable by grepping AppProviders.tsx, which
excludes anyone who doesn't already know the devtools exist. The README now
names the flag next to the rest of the webapp's dev setup, and says why the
tool is off by default.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: FelixTJDietrich

standalone/webapp/README.md

@@ -6,7 +6,24 @@ Part of the Apollon monorepo — run it from the repo root with `pnpm dev`, not
 
 ## Stack
 
-React 19, TypeScript, Vite, the shadcn-style [`@tumaet/ui`](../../packages/ui) design system (Base UI primitives + Tailwind v4), Storybook, Vitest, Playwright (visual + e2e).
+React 19, TypeScript, Vite, the shadcn-style [`@tumaet/ui`](../../packages/ui) design system (Base UI primitives + Tailwind v4), [TanStack Query](https://tanstack.com/query) for server state, Storybook, Vitest, Playwright (visual + e2e).
+
+## Debugging server state
+
+HTTP data (diagram loads, version history, version snapshots) goes through
+TanStack Query — see [`src/queries`](src/queries) and the boundary note in
+[`src/queryClient.ts`](src/queryClient.ts).
+
+The Query Devtools are **off by default**: their floating button sits
+bottom-right, on top of the editor's minimap, and every other corner is taken
+by the editor's own chrome. Enable them per browser from the console, then
+reload:
+
+```js
+localStorage.setItem("apollon:query-devtools", "1")
+```
+
+They are stripped from production builds regardless.
 
 ## Scripts