build: add build:web script + pin Node for Cloudflare web deploy

clients/web imports @cumulusvpn/core via workspace:* (its built dist), so the
web build must build core first. Root `yarn build` can't be used by Cloudflare
because `foreach` also builds the desktop (Tauri/Rust) workspace, which fails in
a JS build image. Add `build:web` (core → web only) as the Cloudflare build
command, and pin Node 22 (.node-version) so the build image matches Vite 8's
requirement.

Author: TheTrunk

.node-version

@@ -0,0 +1 @@
+22

package.json

@@ -11,6 +11,7 @@
   ],
   "scripts": {
     "build": "yarn workspaces foreach -Apt run build",
+    "build:web": "yarn workspace @cumulusvpn/core run build && yarn workspace @cumulusvpn/web run build",
     "typecheck": "yarn workspaces foreach -Apt run typecheck",
     "test": "yarn workspaces foreach -Apt run test",
     "lint": "yarn workspaces foreach -Apt run lint",