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/BLEAnnounceHandlingPolicyTests.swift

@@ -213,6 +213,23 @@ struct BLEAnnounceHandlingPolicyTests {
         #expect(decision == .verified)
     }
 
+    @Test
+    func trustPolicyRejectsSigningKeyReplacementAfterNoiseBinding() {
+        let noiseKey = Data(repeating: 0xCC, count: 32)
+        let boundSigningKey = Data(repeating: 0x11, count: 32)
+
+        let decision = BLEAnnounceTrustPolicy.evaluate(
+            hasSignature: true,
+            signatureValid: true,
+            existingNoisePublicKey: noiseKey,
+            announcedNoisePublicKey: noiseKey,
+            authenticatedSigningPublicKey: boundSigningKey,
+            announcedSigningPublicKey: Data(repeating: 0x22, count: 32)
+        )
+
+        #expect(decision == .reject(.authenticatedSigningKeyMismatch))
+    }
+
     @Test
     func responsePolicyConnectsOnlyForDirectNewOrReconnectedPeers() {
         let directNew = BLEAnnounceResponsePolicy.plan(