ext: MV3 manifest + esbuild build; drop polyfill for Firefox's browser global

Firefox-only extension, so webextension-polyfill (a Chrome shim) is dead
weight and forces a bundler just to resolve one bare import. Use the native
`browser.*` global with @types/firefox-webext-browser instead.

  - manifest.json: MV3, event-page background (dist/background.js), the
    docs/05 §7 permission set (<all_urls> in host_permissions),
    strict_min_version 128.0, data_collection_permissions none.
  - scripts/build.mjs: esbuild bundle of src/background/index.ts -> dist/,
    esm, target firefox128. Wired to `prepare` so `npm ci` produces the
    bundle and CI's `web-ext lint` (which needs it to exist) passes with no
    added CI step. dist/ stays gitignored.
  - src/background/index.ts: event-page entry — brings the transport up,
    holds the shared reference. Capture surfaces attach here next.
  - transport/storage.ts, transport/index.ts: use the browser global.
  - tests/setup.ts: stub the browser global instead of mocking a module.

web-ext lint clean (0/0/0). typecheck clean. 38 tests still green.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_012Y9RU58hD1BuwP82DySUHk

extension/package.json

@@ -5,17 +5,17 @@
   "description": "Velox Firefox extension (lane EXT). Collects URL + headers + cookies and hands downloads to veloxd; renders daemon state. Zero download logic.",
   "type": "module",
   "scripts": {
+    "build": "node scripts/build.mjs",
+    "prepare": "node scripts/build.mjs",
     "typecheck": "tsc --noEmit",
     "test": "vitest run",
     "test:watch": "vitest",
     "lint": "web-ext lint --source-dir ."
   },
-  "dependencies": {
-    "webextension-polyfill": "^0.12.0"
-  },
   "devDependencies": {
-    "@types/webextension-polyfill": "^0.12.1",
+    "@types/firefox-webext-browser": "^120.0.4",
     "@types/ws": "^8.5.12",
+    "esbuild": "^0.24.0",
     "typescript": "^5.6.0",
     "vitest": "^2.1.0",
     "web-ext": "^8.3.0",