Implement browser extension MVP: autofill, save prompt, popup UI ...

Features:
- Content script detects login forms and shows fixed autofill cards
- Persistent save prompt via background storage + popup badge
- Popup built entirely with gnexus-ui-kit components
- Save deduplication by username+domain
- Tags and category (web) for saved credentials
- Manual refresh button to invalidate cache

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>