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/SecureIdentityStateManagerTests.swift

@@ -493,6 +493,59 @@ final class SecureIdentityStateManagerTests: XCTestCase {
         XCTAssertTrue(cleared)
     }
 
+    func test_privateMediaCapabilityPinPersistsMonotonicallyAndPanicClearRemovesIt() async {
+        let keychain = MockKeychain()
+        let fingerprint = Data(repeating: 0x42, count: 32).sha256Fingerprint()
+        let manager = SecureIdentityStateManager(keychain)
+
+        XCTAssertFalse(manager.hasObservedPrivateMediaCapability(fingerprint: fingerprint))
+        manager.markPrivateMediaCapable(fingerprint: fingerprint)
+        XCTAssertTrue(
+            manager.hasObservedPrivateMediaCapability(fingerprint: fingerprint),
+            "pin insertion must be synchronously visible to the next downgrade decision"
+        )
+
+        // Re-marking is idempotent, and the encrypted cache carries the pin
+        // across launches.
+        manager.markPrivateMediaCapable(fingerprint: fingerprint)
+        manager.forceSave()
+        let reloaded = SecureIdentityStateManager(keychain)
+        XCTAssertTrue(reloaded.hasObservedPrivateMediaCapability(fingerprint: fingerprint))
+
+        // ChatViewModel's panic path calls this same wipe after deleting
+        // keychain data; the in-memory pin must disappear immediately too.
+        reloaded.clearAllIdentityData()
+        let cleared = await waitUntil {
+            !reloaded.hasObservedPrivateMediaCapability(fingerprint: fingerprint)
+        }
+        XCTAssertTrue(cleared)
+    }
+
+    func test_noiseAuthenticatedSigningKeyBindingPersistsAndPanicClearRemovesIt() async {
+        let keychain = MockKeychain()
+        let fingerprint = Data(repeating: 0x31, count: 32).sha256Fingerprint()
+        let firstKey = Data(repeating: 0x41, count: 32)
+        let rotatedKey = Data(repeating: 0x42, count: 32)
+        let manager = SecureIdentityStateManager(keychain)
+
+        manager.bindAuthenticatedSigningPublicKey(firstKey, fingerprint: fingerprint)
+        XCTAssertEqual(manager.authenticatedSigningPublicKey(forFingerprint: fingerprint), firstKey)
+        // A later authenticated Noise session may legitimately rotate the
+        // announcement signing key.
+        manager.bindAuthenticatedSigningPublicKey(rotatedKey, fingerprint: fingerprint)
+        XCTAssertEqual(manager.authenticatedSigningPublicKey(forFingerprint: fingerprint), rotatedKey)
+
+        manager.forceSave()
+        let reloaded = SecureIdentityStateManager(keychain)
+        XCTAssertEqual(reloaded.authenticatedSigningPublicKey(forFingerprint: fingerprint), rotatedKey)
+
+        reloaded.clearAllIdentityData()
+        let cleared = await waitUntil {
+            reloaded.authenticatedSigningPublicKey(forFingerprint: fingerprint) == nil
+        }
+        XCTAssertTrue(cleared)
+    }
+
     func test_forceSave_withFailingCacheWriteDoesNotPersistCache() async {
         let keychain = FailingCacheSaveKeychain()
         let manager = SecureIdentityStateManager(keychain)