finish SwiftData migration with @Query views

- Drive chat list, timeline and forward picker from SwiftData @Query
  instead of AppModel's in-memory arrays; inject the per-account
  ModelContext from RootView with an in-memory fallback.
- Physically delete locally deleted messages (context.delete) and
  purge rows marked deleted by older builds on store open, so @Query
  views never resurrect them.
- Delete the legacy JSON snapshot only after a successful import save
  and restore the completeUntilFirstUserAuthentication data protection
  attribute on the store file.
- Mirror the old ChatArchive tolerant decoding in LegacyArchiveImporter
  so snapshots from older schema versions (missing reactions,
  isGroupMessage, roster fields) still import.
- Cover the new behaviour in ArchiveStoreTests and guard it with new
  verify.sh invariants; update AGENTS.md, ARCHITECTURE.md and
  SECURITY.md.

Sources/Shared/UI/MainChatView.swift

@@ -1,4 +1,5 @@
 import Foundation
+import SwiftData
 import SwiftUI
 
 struct MainChatView: View {
@@ -21,27 +22,45 @@ struct MainChatView: View {
     @State private var showingNewGroup = false
     @State private var showingSettings = false
 
+    /// SwiftData-backed chat list. The query orders by last activity;
+    /// `filteredConversations` applies the pinned-first ordering that matches
+    /// `AppModel.sortConversations` (`Bool` is not `Comparable`, so pinning
+    /// cannot be a `SortDescriptor`).
+    @Query(
+        sort: [
+            SortDescriptor(\Conversation.lastActivity, order: .reverse),
+        ]
+    )
+    private var conversations: [Conversation]
+
     private var filteredConversations: [Conversation] {
-        guard !searchText.isEmpty else { return model.conversations }
-        return model.conversations.filter(matchesSearch)
+        let source = searchText.isEmpty ? conversations : conversations.filter(matchesSearch)
+        return source.sorted(by: conversationSort)
+    }
+
+    private func conversationSort(_ lhs: Conversation, _ rhs: Conversation) -> Bool {
+        if lhs.isPinned != rhs.isPinned { return lhs.isPinned }
+        if lhs.lastActivity != rhs.lastActivity { return lhs.lastActivity > rhs.lastActivity }
+        return lhs.displayName.localizedCaseInsensitiveCompare(rhs.displayName)
+            == .orderedAscending
     }
 
     private var filteredRosterContacts: [Conversation] {
-        model.conversations
+        conversations
             .filter { !$0.isGroup && model.rosterContactJIDs.contains($0.jid) }
             .filter { searchText.isEmpty || matchesSearch($0) }
             .sorted(by: contactSort)
     }
 
     private var filteredGroupContacts: [Conversation] {
-        model.conversations
+        conversations
             .filter { $0.isGroup }
             .filter { searchText.isEmpty || matchesSearch($0) }
             .sorted(by: contactSort)
     }
 
     private var hasStoredContacts: Bool {
-        !model.rosterContactJIDs.isEmpty || model.conversations.contains { $0.isGroup }
+        !model.rosterContactJIDs.isEmpty || conversations.contains { $0.isGroup }
     }
 
     var body: some View {