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/Views/ContentView.swift

@@ -1188,8 +1188,7 @@ struct ContentView: View {
                         showLocationNotes = true
                     }) {
                         HStack(alignment: .center, spacing: 4) {
-                            let currentCount = (notesCounter.count ?? 0)
-                            let hasNotes = (!notesCounter.initialLoadComplete ? max(currentCount, sheetNotesCount) : currentCount) > 0
+                            let hasNotes = (notesCounter.count ?? 0) > 0
                             Image(systemName: "long.text.page.and.pencil")
                                 .font(.bitchatSystem(size: 12))
                                 .foregroundColor(hasNotes ? textColor : Color.gray)
@@ -1290,7 +1289,9 @@ struct ContentView: View {
                 .onAppear { viewModel.isLocationChannelsSheetPresented = true }
                 .onDisappear { viewModel.isLocationChannelsSheetPresented = false }
         }
-        .sheet(isPresented: $showLocationNotes) {
+        .sheet(isPresented: $showLocationNotes, onDismiss: {
+            notesGeohash = nil
+        }) {
             Group {
                 if let gh = notesGeohash ?? LocationChannelManager.shared.availableChannels.first(where: { $0.level == .building })?.geohash {
                     LocationNotesView(geohash: gh, onNotesCountChanged: { cnt in sheetNotesCount = cnt })