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,

Sources/Shared/UI/Components/PhotoAttachmentPreview.swift

@@ -124,7 +124,7 @@ struct PhotoAttachmentPreview: View {
     private func openViewer() {
         Task {
             await model.presentMediaViewer(message)
-            if model.mediaViewerItem?.id != message.id,
+            if model.mediaViewerItem?.id != message.clientID,
                message.remoteAttachmentURL == nil {
                 onFallbackTap()
             }

Sources/Shared/UI/Components/ReplyThreadOverlay.swift

@@ -112,10 +112,10 @@ struct ReplyThreadOverlay: View {
                     model: model,
                     message: reply,
                     maximumBubbleWidth: maximumBubbleWidth,
-                    isSelected: reply.id == selectedReplyID,
-                    showsDeliveryStatus: reply.id == selectedReplyID
+                    isSelected: reply.clientID == selectedReplyID,
+                    showsDeliveryStatus: reply.clientID == selectedReplyID
                 )
-                .id(reply.id)
+                .id(reply.clientID)
             }
         }
         .padding(.leading, showsBracket && bracketOnLeadingEdge ? 43 : 0)

Sources/Shared/UI/Components/VideoAttachmentPreview.swift

@@ -142,7 +142,7 @@ struct VideoAttachmentPreview: View {
     private func openViewer() {
         Task {
             await model.presentMediaViewer(message)
-            if model.mediaViewerItem?.id != message.id,
+            if model.mediaViewerItem?.id != message.clientID,
                message.remoteAttachmentURL == nil {
                 onFallbackTap()
             }

Sources/Shared/UI/Components/VideoNotePreview.swift

@@ -82,7 +82,7 @@ struct VideoNotePreview: View {
         }
         .onReceive(NotificationCenter.default.publisher(for: .lumaExclusiveMediaPlayback)) { notification in
             guard let activeID = notification.object as? String,
-                  activeID != message.id else { return }
+                  activeID != message.clientID else { return }
             player?.pause()
             isPlaying = false
         }
@@ -146,7 +146,7 @@ struct VideoNotePreview: View {
             model.audioPlayback.stop()
             NotificationCenter.default.post(
                 name: .lumaExclusiveMediaPlayback,
-                object: message.id
+                object: message.clientID
             )
             player.play()
         }