Initial scaffold: Beat Pocket MV3 extension

- manifest.json with MV3 config, host permissions for beat-battle.net
- content/detector.ts: route + overlay observer, emits bbp:phase events
- content/layout.ts: defensive mobile-first layout reflow
- content/chat-drawer.ts: bottom-sheet drawer, dispatches synthetic events
  through site's own React-managed textarea + button (preserves auth,
  rate-limit, mute, profanity guards)
- content/util/selectors.ts: text- and role-based fallback chain
- content/util/ws-proxy.ts: lightweight WebSocket instrumentation, no
  payload capture
- content/inject.css: mobile overrides using site's existing --bb-* vars
- background.js: MV3 service worker stub
- package.json + tsconfig.json + esbuild.config.mjs: build pipeline
- icons/: 16/32/48/128 placeholder PNGs (chili-red disc)
- docs/PLAN.md: full V1 architecture, risks, scope
- README.md: scope, dev workflow (load unpacked), tech

extension/background.js

@@ -0,0 +1,13 @@
+// Minimal service worker for MV3. Mostly exists to be valid.
+// (Future: track badge state for unread chat messages.)
+chrome.runtime.onInstalled.addListener(() => {
+  // no-op
+});
+
+chrome.runtime.onMessage.addListener((msg, _sender, sendResponse) => {
+  if (msg?.type === 'bbp:ping') {
+    sendResponse({ ok: true });
+    return true;
+  }
+  return false;
+});