Cross-browser release — MTD now works on both Firefox and Chrome.

Changes from v1.0:
- Fixed badge counter (was always empty due to a bug in IndexedDB reads)
- Fixed Moodle detection returning undefined instead of false
- Null-safe fallbacks for missing test name or subject
- Pending test data persisted to storage.local, survives service worker restarts
- Removed unused permissions (notifications, tabs, downloads)