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/Models/ChatTimelineEntry.swift

@@ -3,11 +3,21 @@ import Foundation
 /// A cached presentation row for the selected chat. Building day boundaries
 /// once per message mutation avoids walking the full history during every
 /// scroll-position or composer-state update.
-struct ChatTimelineEntry: Identifiable, Hashable, Sendable {
+struct ChatTimelineEntry: Identifiable, Hashable {
     let message: ChatMessage
     let startsNewDay: Bool
 
-    var id: String { message.id }
+    var id: String { message.clientID }
+
+    static func == (lhs: Self, rhs: Self) -> Bool {
+        lhs.message.clientID == rhs.message.clientID
+            && lhs.startsNewDay == rhs.startsNewDay
+    }
+
+    func hash(into hasher: inout Hasher) {
+        hasher.combine(message.clientID)
+        hasher.combine(startsNewDay)
+    }
 
     static func make(
         from messages: [ChatMessage],