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/tests/setup.ts

@@ -1,9 +1,9 @@
-// Global test setup: a mock of `webextension-polyfill` with an in-memory
-// storage.local, so storage.ts and the transport picker run without a browser.
+// Global test setup: a minimal stand-in for Firefox's `browser` global with an
+// in-memory storage.local, so storage.ts and the transport picker run under vitest.
 
-import { beforeEach, vi } from 'vitest';
+import { beforeEach } from 'vitest';
 
-vi.mock('webextension-polyfill', () => {
+function makeBrowserShim() {
   const store = new Map<string, unknown>();
   const local = {
     get: async (keys?: string | string[] | Record<string, unknown> | null) => {
@@ -25,20 +25,19 @@ vi.mock('webextension-polyfill', () => {
     },
   };
   return {
-    default: {
-      storage: { local },
-      runtime: {
-        getURL: (path = '/') =>
-          `moz-extension://11111111-2222-3333-4444-555555555555${path}`,
-        connectNative: () => {
-          throw new Error('browser.runtime.connectNative was not stubbed for this test');
-        },
+    storage: { local },
+    runtime: {
+      getURL: (path = '/') => `moz-extension://11111111-2222-3333-4444-555555555555${path}`,
+      connectNative: () => {
+        throw new Error('browser.runtime.connectNative was not stubbed for this test');
       },
     },
   };
-});
+}
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+(globalThis as any).browser = makeBrowserShim();
 
 beforeEach(async () => {
-  const browser = (await import('webextension-polyfill')).default;
   await browser.storage.local.clear();
 });

extension/tests/transport/index.test.ts

@@ -1,6 +1,5 @@
 import { afterEach, describe, expect, it } from 'vitest';
 import { WebSocket as WsClient } from 'ws';
-import browser from 'webextension-polyfill';
 
 import { createTransport } from '../../src/background/transport/index.js';
 import type { WebSocketCtor } from '../../src/background/transport/websocket.js';