Add plus-minus feature to location notes (± 1 grid) (#821)

Expands location notes coverage to include 8 neighboring geohash cells,
creating a 3×3 grid around the user's current building-level location.

Changes:
- Add Geohash.neighbors() method to calculate 8 surrounding cells
- Update LocationNotesManager to subscribe to center + neighbors (9 cells total)
- Add NostrFilter.geohashNotes([String]) overload for multi-cell subscriptions
- Display '± 1' indicator in LocationNotesView header

Coverage:
- Single cell: ~38m × 19m
- 3×3 grid: ~114m × 57m total area
- Better discovery of nearby activity

Behavior:
- Subscribe: Fetches notes from all 9 cells (single efficient subscription)
- Post: Still uses center geohash only (correct privacy-preserving behavior)
- UI: Shows 'geohash ± 1' to indicate expanded coverage

Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchat/Nostr/NostrRelayManager.swift

@@ -906,6 +906,16 @@ struct NostrFilter: Encodable {
         filter.limit = limit
         return filter
     }
+
+    // For location notes with neighbors: subscribe to multiple geohashes (center + neighbors)
+    static func geohashNotes(_ geohashes: [String], since: Date? = nil, limit: Int = 200) -> NostrFilter {
+        var filter = NostrFilter()
+        filter.kinds = [1]
+        filter.since = since?.timeIntervalSince1970.toInt()
+        filter.tagFilters = ["g": geohashes]
+        filter.limit = limit
+        return filter
+    }
 }
 
 // Dynamic coding key for tag filters