* Add geohash bookmarks: persistence, sampling integration, and UI\n\n- Add GeohashBookmarksStore with UserDefaults persistence and toggle API\n- Sample union of regional + bookmarked geohashes for activity notifications\n- LocationChannelsSheet: bookmark icons on nearby rows and a Bookmarked section\n- Header toolbar: toggle bookmark for current geohash\n- Tests: GeohashBookmarksStoreTests for normalization and persistence\n\nRationale: Bookmarked geohashes are always sampled for new activity notifications and quickly selectable from the sheet.
* Notifications: remove background 'new chats!' nudge; geohash activity only on zero→alive; mesh 'nearby' only on zero→alive\n\n- Geohash sampling: notify only when previous participant count in last 5m was zero, with 30s freshness gate\n- Suppress old sampled events after (re)subscribe\n- Remove channel inactivity nudge\n- Mesh: reset rising-edge gate immediately when peer list goes empty (strict zero→alive)
* Geohash notifications: ensure triggering message appears\n\n- On zero→alive sampling, pre-populate geoTimelines[gh] with the triggering message\n- Dedup on per-geohash timeline and UI messages by message ID to avoid duplicates after subscribe\n- Keeps participant update, block/self checks, and cooldown intact
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Co-authored-by: jack <jackjackbits@users.noreply.github.com>
