Location notes: fix performance and UI issues (#774)

* Location notes: fix performance and UI issues

Performance fixes:
- Add Set-based duplicate detection (O(1) vs O(n) lookup)
- Eliminates lag when receiving 200+ notes during EOSE

Correctness fixes:
- Fix optimistic echo timestamp to match signed event timestamp
- Add echo IDs to noteIDs set for consistency

UI improvements:
- Remove duplicate "loading recent notes" text in header
- Simplify toolbar icon color logic for immediate green indication
- Icon now reliably turns green when notes exist in geohash

Tests: All 3 LocationNotes tests passing

* Location notes: add remaining robustness fixes

Additional improvements:
- Align counter/manager limits to 200 (prevents showing count higher than displayable)
- Set loading state before clearing notes to eliminate UI flicker on geohash change
- Add geohash validation for building-level precision (8 valid base32 chars)
- Add defensive 500-note memory cap with automatic trimming
- Clear stale notesGeohash state on sheet dismiss

Tests:
- Fix test geohashes to use valid base32 characters
- All 3 LocationNotes tests passing

---------

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

bitchat/Protocols/Geohash.swift

@@ -10,6 +10,14 @@ enum Geohash {
         return map
     }()
 
+    /// Validates a geohash string for building-level precision (8 characters).
+    /// - Parameter geohash: The geohash string to validate
+    /// - Returns: true if valid 8-character base32 geohash, false otherwise
+    static func isValidBuildingGeohash(_ geohash: String) -> Bool {
+        guard geohash.count == 8 else { return false }
+        return geohash.lowercased().allSatisfy { base32Map[$0] != nil }
+    }
+
     /// Encodes the provided coordinates into a geohash string.
     /// - Parameters:
     ///   - latitude: Latitude in degrees (-90...90)