Mesh robustness optimizations (#463)

* Mesh robustness: link-aware fragmentation, relay jitter, connection budget, adaptive scanning; fix BLE queue deadlock

- Link-aware fragmentation using per-link write/notify limits
- Directed fragments for one-to-one; exclude from relays
- Add relay jitter with cancel-on-duplicate to reduce floods
- Cap central links and rate-limit connect attempts with queue
- Foreground duty-cycled scanning when connected; continuous when isolated
- Fix deadlock by avoiding sync on BLE queue when already on it
- Silence Keychain var->let warnings

* Mesh: add relay jitter; dynamic RSSI threshold; candidate scoring/backoff; fix misplaced lines in BLEService extension

* macOS UI: restore mesh peer list when geohash feature is iOS-only by moving mesh list outside iOS switch (ContentView)

* ContentView: add macOS mesh peer list under #else to fix missing People section on macOS

* Refactor People section: extract MeshPeerList and GeohashPeopleList into separate views to reduce SwiftUI type-checking complexity; integrate in ContentView

* BLE: prevent re-fragmentation loops; UI: darker mesh blue + first-row spacing in peer list

* Mesh flood/battery: probabilistic relays, adaptive TTL caps, fragment pacing + scan pause, assembly cap; enable relayed DMs with direct-first then flood fallback.

* Refactor BLEService broadcast: split pad policy, encrypted/direct-first handler, and link-aware sender; simplify broadcastPacket to delegate by type.

* Extract relay decision into RelayController with RelayDecision; simplify handleReceivedPacket to use it.

* Move RelayController and RelayDecision into their own file under Services; keep BLEService leaner.

* UI: adjust mesh blue to a darker, less purple tone; apply consistently for count and #mesh badge.

* Project: include RelayController.swift in target and sync project file changes.

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

bitchat/Services/KeychainManager.swift

@@ -127,7 +127,7 @@ class KeychainManager {
     
     private func retrieveData(forKey key: String) -> Data? {
         // Base query
-        var base: [String: Any] = [
+        let base: [String: Any] = [
             kSecClass as String: kSecClassGenericPassword,
             kSecAttrAccount as String: key,
             kSecAttrService as String: service,
@@ -158,7 +158,7 @@ class KeychainManager {
     
     private func delete(forKey key: String) -> Bool {
         // Base delete query
-        var base: [String: Any] = [
+        let base: [String: Any] = [
             kSecClass as String: kSecClassGenericPassword,
             kSecAttrAccount as String: key,
             kSecAttrService as String: service