Remove noise service exposure; single-owner selection state

Transport callers no longer reach the raw NoiseEncryptionService:
getNoiseService() is deleted in favor of narrow purpose-named Transport
methods (session public key, identity fingerprint, static/signing keys,
sign/verify, callback installation). VerificationService now reaches
crypto through the transport, so it can no longer pin a stale service
across a panic reset. myPeerID/myNickname become private(set); the
existing setNickname mutator is the sole nickname path.

ConversationStore is now the sole owner of private-chat selection:
PrivateChatManager.selectedPeer is a published read-only mirror, and
startChat/endChat mutate through the store intent. The bridge method
and its five call sites are deleted, removing a latent bug where a
stale manager selection pushed back into the store could resurrect a
just-removed conversation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bitchatTests/Mocks/MockBLEService.swift

@@ -34,9 +34,7 @@ final class MockBLEService: NSObject {
     weak var delegate: BitchatDelegate?
     var myPeerID = PeerID(str: "MOCK1234")
     var myNickname: String = "MockUser"
-    
-    private let mockKeychain = MockKeychain()
-    
+
     // Test-specific properties
     var sentMessages: [(message: BitchatMessage, packet: BitchatPacket)] = []
     var sentPackets: [BitchatPacket] = []
@@ -240,10 +238,6 @@ final class MockBLEService: NSObject {
         delegate?.didUpdatePeerList([])
     }
     
-    func getNoiseService() -> NoiseEncryptionService {
-        return NoiseEncryptionService(keychain: mockKeychain)
-    }
-    
     func getFingerprint(for peerID: String) -> String? {
         return nil
     }