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/RootView.swift

@@ -1,4 +1,5 @@
 import QuickLook
+import SwiftData
 import SwiftUI
 
 struct RootView: View {
@@ -11,6 +12,7 @@ struct RootView: View {
                     LoginView(model: model)
                 } else {
                     MainChatView(model: model)
+                        .environment(\.modelContext, model.modelContext)
                 }
             }