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/manifest.json

@@ -0,0 +1,34 @@
+{
+  "manifest_version": 3,
+  "name": "Velox",
+  "version": "0.1.0",
+  "description": "Hands Firefox downloads to the Velox download manager. Collects the URL, headers and cookies; the daemon does the rest.",
+
+  "browser_specific_settings": {
+    "gecko": {
+      "id": "[email protected]",
+      "strict_min_version": "128.0",
+      "data_collection_permissions": {
+        "required": ["none"]
+      }
+    }
+  },
+
+  "background": {
+    "scripts": ["dist/background.js"],
+    "type": "module"
+  },
+
+  "permissions": [
+    "webRequest",
+    "webRequestBlocking",
+    "downloads",
+    "cookies",
+    "contextMenus",
+    "storage",
+    "notifications",
+    "nativeMessaging"
+  ],
+
+  "host_permissions": ["<all_urls>"]
+}