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>

bitchat/ViewModels/ChatVerificationCoordinator.swift

@@ -98,13 +98,14 @@ extension ChatViewModel: ChatVerificationContext {
         onPeerAuthenticated: @escaping (PeerID, String) -> Void,
         onHandshakeRequired: @escaping (PeerID) -> Void
     ) {
-        let noiseService = meshService.getNoiseService()
-        noiseService.onPeerAuthenticated = onPeerAuthenticated
-        noiseService.onHandshakeRequired = onHandshakeRequired
+        meshService.installNoiseSessionCallbacks(
+            onPeerAuthenticated: onPeerAuthenticated,
+            onHandshakeRequired: onHandshakeRequired
+        )
     }
 
     func noiseStaticPublicKeyData() -> Data {
-        meshService.getNoiseService().getStaticPublicKeyData()
+        meshService.noiseStaticPublicKeyData()
     }
 
     func sendVerifyChallenge(to peerID: PeerID, noiseKeyHex: String, nonceA: Data) {