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/TransferProgressManagerTests.swift

@@ -49,7 +49,7 @@ struct TransferProgressManagerTests {
                 recorder.append("updated:\(id):\(sent):\(total)")
             case .completed(let id, let total):
                 recorder.append("completed:\(id):\(total)")
-            case .cancelled:
+            case .cancelled, .rejected:
                 break
             }
         }
@@ -85,7 +85,7 @@ struct TransferProgressManagerTests {
                 recorder.append("started:\(id):\(total)")
             case .cancelled(let id, let sent, let total):
                 recorder.append("cancelled:\(id):\(sent):\(total)")
-            case .updated, .completed:
+            case .updated, .completed, .rejected:
                 break
             }
         }
@@ -105,6 +105,28 @@ struct TransferProgressManagerTests {
         #expect(manager.snapshot(id: transferID) == nil)
         _ = cancellable
     }
+
+    @Test("Preflight policy rejection publishes a visible failure reason")
+    @MainActor
+    func rejectBeforeStartPublishesReason() async {
+        let manager = TransferProgressManager()
+        let transferID = "transfer-visible-reject"
+        let recorder = EventRecorder()
+        let cancellable = manager.publisher.sink { event in
+            if case .rejected(let id, let reason) = event {
+                recorder.append("rejected:\(id):\(reason)")
+            }
+        }
+
+        manager.rejectBeforeStart(id: transferID, reason: "upgrade required")
+
+        let didReceive = await TestHelpers.waitUntil({
+            recorder.values == ["rejected:\(transferID):upgrade required"]
+        }, timeout: 5.0)
+        #expect(didReceive)
+        #expect(manager.snapshot(id: transferID) == nil)
+        _ = cancellable
+    }
 }
 
 private final class EventRecorder: @unchecked Sendable {