migrate persistence from JSON to SwiftData

Models:
- Convert Conversation and ChatMessage to SwiftData @Model classes with
  a
  one-to-many relationship and cascade delete.
- Rename ChatMessage.id to clientID and drop Conversation.id in favor of
  jid
  (the string id would clash with PersistentIdentifier).
- Keep MessageReaction as a Codable value stored in an array attribute.

Persistence:
- Add ArchiveStore, a per-account ModelContainer/ModelContext that
  replaces
  the JSON ChatArchive with load/save/erase.
- Add ArchiveMetadataRecord for the durable MAM checkpoints, cursor,
  locally
  deleted message IDs and roster contact JIDs.
- Add LegacyArchiveImporter to import the old JSON snapshot once and
  delete
  the file afterwards, preserving existing history.

AppModel:
- Replace ChatArchive with ArchiveStore and persist via context.save();
  insert new models and delete the replaced object on upsert merges so
  SwiftData never keeps an orphaned duplicate.

Tests:
- Replace ChatArchiveTests with ArchiveStoreTests (store round-trip and
  legacy import) and drop the now-obsolete Codable round-trip
  assertions.

Sources/Shared/UI/Components/AudioMessagePlayer.swift

@@ -129,7 +129,7 @@ struct AudioMessagePlayer: View {
     }
 
     private var isCurrent: Bool {
-        playback.currentMessageID == message.id
+        playback.currentMessageID == message.clientID
     }
 
     private var isPlaying: Bool {
@@ -194,7 +194,7 @@ struct AudioMessagePlayer: View {
                     return
                 }
                 playback.toggle(
-                    messageID: message.id,
+                    messageID: message.clientID,
                     url: downloadedURL,
                     durationHint: message.duration,
                     isVoice: message.kind == .voice
@@ -203,7 +203,7 @@ struct AudioMessagePlayer: View {
             return
         }
         playback.toggle(
-            messageID: message.id,
+            messageID: message.clientID,
             url: localURL,
             durationHint: message.duration,
             isVoice: message.kind == .voice
@@ -213,7 +213,7 @@ struct AudioMessagePlayer: View {
     private func seek(_ fraction: Double) {
         guard let localURL else { return }
         playback.seek(
-            messageID: message.id,
+            messageID: message.clientID,
             url: localURL,
             durationHint: message.duration,
             isVoice: message.kind == .voice,