feat(extension): build and ship a Firefox add-on (#188)

Derive a Gecko-compatible manifest at build time and package
extension-firefox.zip alongside the Chrome zip:

- background service worker is declared as an event-page `scripts`
  entry (top-level listeners + in-memory Map run unchanged on Gecko)
- browser_specific_settings.gecko with id, strict_min_version 140.0,
  and data_collection_permissions (required by AMO; honored on 140+)
- packZip helper parameterized over cwd/excludes; `*.DS_Store` strips
  junk at every depth and .DS_Store is excluded from the staging copy
- guard against a missing background.service_worker shape

CI now builds the extension and runs a pinned `web-ext@8 lint` over
the staged Firefox tree (innerHTML warnings are non-blocking); the
unpacked staging dir is excluded from the uploaded artifact. The
release script attaches both zips and points to AMO.

Bumps the extension to v1.2.0 with a changelog entry.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Paul Bakaus <paul.bakaus@gmail.com>

extension/manifest.json

@@ -2,7 +2,7 @@
   "manifest_version": 3,
   "name": "Impeccable",
   "description": "Detect common UI anti-patterns in any web page",
-  "version": "1.1.0",
+  "version": "1.2.0",
   "permissions": ["activeTab", "scripting", "storage", "webNavigation"],
   "host_permissions": ["<all_urls>"],
   "background": {