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/BLEFragmentAssemblyBufferTests.swift

@@ -117,6 +117,35 @@ struct BLEFragmentAssemblyBufferTests {
         }
     }
 
+    @Test
+    func encryptedPrivateFileAssemblyGetsFramedFileHeadroom() throws {
+        var buffer = BLEFragmentAssemblyBuffer()
+        let fragmentID = Data(repeating: 0x15, count: 8)
+        let first = try #require(BLEFragmentHeader(packet: makeFragmentPacket(
+            fragmentID: fragmentID,
+            index: 0,
+            total: 2,
+            originalType: MessageType.noiseEncrypted.rawValue,
+            fragmentData: Data(repeating: 0x01, count: FileTransferLimits.maxPayloadBytes)
+        )))
+        let second = try #require(BLEFragmentHeader(packet: makeFragmentPacket(
+            fragmentID: fragmentID,
+            index: 1,
+            total: 2,
+            originalType: MessageType.noiseEncrypted.rawValue,
+            fragmentData: Data([0x02])
+        )))
+
+        _ = buffer.append(first, maxInFlightAssemblies: 8)
+        let result = buffer.append(second, maxInFlightAssemblies: 8)
+
+        if case let .complete(_, data, _) = result {
+            #expect(data.count == FileTransferLimits.maxPayloadBytes + 1)
+        } else {
+            Issue.record("Expected encrypted private-file assembly to use framed-file limit")
+        }
+    }
+
     @Test
     func removeExpiredDropsOldAssemblies() throws {
         var buffer = BLEFragmentAssemblyBuffer()