feat: mute thread notifications

[barrydeen]

Summary

• Adds a "Mute Thread" option to the 3-dot menu on notification PostCards
• Muted thread root IDs stored locally in MuteRepository (SharedPreferences), persists across sessions
• Incoming notifications from muted threads are filtered in NotificationRepository.addEvent()
• Existing notification groups for the thread are purged immediately on mute
• Notes remain visible when browsing the thread directly via feed — only notifications are suppressed

Test plan

• Open notifications screen, tap 3-dot menu on any notification — "Mute Thread" appears
• Tap "Mute Thread" — notification group disappears from list
• New notifications from that thread do not appear
• Navigate to the thread directly via feed — all notes still visible
• Kill and restart app — muted thread persists