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/MockTransport.swift

@@ -109,8 +109,34 @@ final class MockTransport: Transport {
         triggeredHandshakes.append(peerID)
     }
 
-    func getNoiseService() -> NoiseEncryptionService {
-        NoiseEncryptionService(keychain: mockKeychain)
+    // Noise identity wrappers backed by a mock-keychain encryption service
+    // (mirrors the previous `getNoiseService()` placeholder behavior: a real
+    // identity, but no peer sessions). Exposed so tests can assert against
+    // the same identity the wrappers use.
+    private(set) lazy var mockNoiseService = NoiseEncryptionService(keychain: mockKeychain)
+
+    func noiseSessionPublicKeyData(for peerID: PeerID) -> Data? {
+        mockNoiseService.getPeerPublicKeyData(peerID)
+    }
+
+    func noiseIdentityFingerprint() -> String {
+        mockNoiseService.getIdentityFingerprint()
+    }
+
+    func noiseStaticPublicKeyData() -> Data {
+        mockNoiseService.getStaticPublicKeyData()
+    }
+
+    func noiseSigningPublicKeyData() -> Data {
+        mockNoiseService.getSigningPublicKeyData()
+    }
+
+    func noiseSignData(_ data: Data) -> Data? {
+        mockNoiseService.signData(data)
+    }
+
+    func noiseVerifySignature(_ signature: Data, for data: Data, publicKey: Data) -> Bool {
+        mockNoiseService.verifySignature(signature, for: data, publicKey: publicKey)
     }
 
     // MARK: - Messaging