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/Protocols/Geohash.swift

@@ -119,4 +119,57 @@ enum Geohash {
         }
         return (latInterval.0, latInterval.1, lonInterval.0, lonInterval.1)
     }
+
+    /// Returns all 8 neighboring geohash cells at the same precision.
+    /// - Parameter geohash: Base32 geohash string.
+    /// - Returns: Array of 8 neighboring geohashes (N, NE, E, SE, S, SW, W, NW order).
+    static func neighbors(of geohash: String) -> [String] {
+        guard !geohash.isEmpty else { return [] }
+
+        let precision = geohash.count
+        let bounds = decodeBounds(geohash)
+        let center = decodeCenter(geohash)
+
+        // Calculate cell dimensions
+        let latHeight = bounds.latMax - bounds.latMin
+        let lonWidth = bounds.lonMax - bounds.lonMin
+
+        // Helper to wrap longitude around ±180
+        func wrapLongitude(_ lon: Double) -> Double {
+            var wrapped = lon
+            while wrapped > 180.0 { wrapped -= 360.0 }
+            while wrapped < -180.0 { wrapped += 360.0 }
+            return wrapped
+        }
+
+        // Helper to clamp latitude to ±90
+        func clampLatitude(_ lat: Double) -> Double {
+            return max(-90.0, min(90.0, lat))
+        }
+
+        // Calculate 8 neighbor centers
+        let neighbors: [(lat: Double, lon: Double)] = [
+            (center.lat + latHeight, center.lon),                    // N
+            (center.lat + latHeight, center.lon + lonWidth),         // NE
+            (center.lat, center.lon + lonWidth),                     // E
+            (center.lat - latHeight, center.lon + lonWidth),         // SE
+            (center.lat - latHeight, center.lon),                    // S
+            (center.lat - latHeight, center.lon - lonWidth),         // SW
+            (center.lat, center.lon - lonWidth),                     // W
+            (center.lat + latHeight, center.lon - lonWidth)          // NW
+        ]
+
+        // Encode each neighbor, handling boundary conditions
+        return neighbors.compactMap { neighbor in
+            let lat = clampLatitude(neighbor.lat)
+            let lon = wrapLongitude(neighbor.lon)
+
+            // Skip if we've crossed a pole (latitude clamped to boundary)
+            if (neighbor.lat > 90.0 || neighbor.lat < -90.0) {
+                return nil
+            }
+
+            return encode(latitude: lat, longitude: lon, precision: precision)
+        }
+    }
 }