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/Services/UnifiedPeerServiceTests.swift

@@ -266,6 +266,11 @@ private final class TestIdentityManager: SecureIdentityStateManagerProtocol {
         verified.removeAll()
     }
 
+    func markPrivateMediaCapable(fingerprint: String) {}
+    func hasObservedPrivateMediaCapability(fingerprint: String) -> Bool { false }
+    func bindAuthenticatedSigningPublicKey(_ signingPublicKey: Data, fingerprint: String) {}
+    func authenticatedSigningPublicKey(forFingerprint fingerprint: String) -> Data? { nil }
+
     func removeEphemeralSession(peerID: PeerID) {}
 
     func setVerified(fingerprint: String, verified: Bool) {