Offer Retry and Remove actions with a reason when tapping a failed message

Tapping the red (!) on a message that failed to send used to show a bare
"Sending failed" alert with only an OK button. Now the alert includes the
failure reason from the SDK (e.g. the HTTP error that caused it) and offers
Retry sending, Remove message and Cancel actions, wired to the send handle's
resend and to redacting the local echo respectively.

ElementX/Sources/Services/Timeline/TimelineItemProxy.swift

@@ -65,7 +65,7 @@ nonisolated enum TimelineItemSendFailure: Hashable {
     }
     
     case verifiedUser(VerifiedUser)
-    case unknown
+    case unknown(reason: String?)
 }
 
 /// A light wrapper around event timeline items returned from Rust.
@@ -95,8 +95,10 @@ final nonisolated class EventTimelineItemProxy: Sendable {
                 return .sendingFailed(.verifiedUser(.changedIdentity(users: users)))
             case .insecureDevices(let userDeviceMap):
                 return .sendingFailed(.verifiedUser(.hasUnsignedDevice(devices: userDeviceMap)))
+            case .genericApiError(let message):
+                return .sendingFailed(.unknown(reason: message))
             default:
-                return .sendingFailed(.unknown)
+                return .sendingFailed(.unknown(reason: nil))
             }
         case .notSentYet:
             return .sending