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/BLEAnnounceHandlerTests.swift

@@ -9,6 +9,7 @@ struct BLEAnnounceHandlerTests {
         var existingSigningPublicKey: Data?
         var persistedSigningPublicKey: Data?
         var persistedSigningKeyQueries: [PeerID] = []
+        var authenticatedSigningPublicKey: Data?
         var signatureValid = true
         var linkState: (hasPeripheral: Bool, hasCentral: Bool) = (false, false)
         var linkBoundToOtherPeer = false
@@ -44,6 +45,7 @@ struct BLEAnnounceHandlerTests {
                 recorder.persistedSigningKeyQueries.append(peerID)
                 return recorder.persistedSigningPublicKey
             },
+            authenticatedSigningPublicKey: { _ in recorder.authenticatedSigningPublicKey },
             verifySignature: { packet, signingPublicKey in
                 recorder.verifySignatureCalls.append((packet, signingPublicKey))
                 return recorder.signatureValid
@@ -703,6 +705,7 @@ struct BLEAnnounceHandlerTests {
                 return (info?.noisePublicKey, info?.signingPublicKey)
             },
             persistedSigningPublicKey: { _ in nil },
+            authenticatedSigningPublicKey: { _ in nil },
             verifySignature: { packet, signingPublicKey in
                 victim.verifyPacketSignature(packet, publicKey: signingPublicKey)
             },
@@ -817,6 +820,7 @@ struct BLEAnnounceHandlerTests {
                         .compactMap { $0.signingPublicKey }
                         .first
                 },
+                authenticatedSigningPublicKey: { _ in nil },
                 verifySignature: { packet, signingPublicKey in
                     victim.verifyPacketSignature(packet, publicKey: signingPublicKey)
                 },