Encrypt private media before BLE fragmentation (#1434)

Closes the last cleartext private-content path over BLE: private DM images/voice were sent as plaintext signed fileTransfer packets, TTL-relayed across the mesh, so every relay saw the full bytes. Now the complete BitchatFilePacket is encrypted as a single Noise AEAD message (inner type 0x20, matching Android) and the opaque ciphertext is fragmented. Adds an authenticated in-session capability proof (0x21 TLV: capabilities + Ed25519 key), TOFU-style downgrade pinning, a per-send consent dialog for the signed-cleartext fallback to legacy peers, and a cancellation/admission registry so cancel/delete cannot race a deferred cleartext send.

Android wire constants (0x20 / 0x21 / capability bit 8) confirmed shipping. The 256-fragment preflight cap applies only to the directed fileTransfer migration fallback; encrypted media to capable peers uses the full receiver ceiling.

Rebased over #1428/#1349: identity reads go through BLELocalIdentityStateStore; the session-bound authenticated signing-key check and the announce-path TOFU pin are kept as complementary checks. Full local suite green (1744+197 tests).

bitchatTests/Mocks/MockIdentityManager.swift

@@ -14,6 +14,8 @@ final class MockIdentityManager: SecureIdentityStateManagerProtocol {
     private var blockedFingerprints: Set<String> = []
     private var blockedNostrPubkeys: Set<String> = []
     private var socialIdentities: [String: SocialIdentity] = [:]
+    private var privateMediaCapableFingerprints: Set<String> = []
+    private var authenticatedSigningKeys: [String: Data] = [:]
 
     init(_: KeychainManagerProtocol) {}
     
@@ -87,7 +89,10 @@ final class MockIdentityManager: SecureIdentityStateManagerProtocol {
     
     func registerEphemeralSession(peerID: PeerID, handshakeState: HandshakeState) {}
 
-    func clearAllIdentityData() {}
+    func clearAllIdentityData() {
+        privateMediaCapableFingerprints.removeAll()
+        authenticatedSigningKeys.removeAll()
+    }
     
     func removeEphemeralSession(peerID: PeerID) {}
     
@@ -101,6 +106,22 @@ final class MockIdentityManager: SecureIdentityStateManagerProtocol {
         Set()
     }
 
+    func markPrivateMediaCapable(fingerprint: String) {
+        privateMediaCapableFingerprints.insert(fingerprint)
+    }
+
+    func hasObservedPrivateMediaCapability(fingerprint: String) -> Bool {
+        privateMediaCapableFingerprints.contains(fingerprint)
+    }
+
+    func bindAuthenticatedSigningPublicKey(_ signingPublicKey: Data, fingerprint: String) {
+        authenticatedSigningKeys[fingerprint] = signingPublicKey
+    }
+
+    func authenticatedSigningPublicKey(forFingerprint fingerprint: String) -> Data? {
+        authenticatedSigningKeys[fingerprint]
+    }
+
     // MARK: Vouching (transitive verification)
 
     private var vouchesByVouchee: [String: [VouchRecord]] = [:]