Message Logger: hidden messages on the server, removed chats, old chat photos

- Hidden deleted messages are marked on the server (local marks migrated);
  "Reveal hidden messages" in the chat menu, Unhide in the message menu and
  the selection bar.
- Chats an account lost are reported, listed among its chats again and
  opened read-only with their logged messages, all deleted.
- Logged chat photos: a removed chat's avatar, and in a chat's photos the
  ones it no longer has, translucent with a trash icon in the viewer's
  previews.
- Fix: the deleted-post comments thread is answered locally again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/config.py

@@ -7,6 +7,7 @@ RESTORE_MISSING = "restore_missing"
 REPORT_DELETIONS = "report_deletions"
 DELETED_ALPHA = "deleted_alpha"
 CACHE_DAYS = "cache_days"
+SHOW_REMOVED_CHATS = "show_removed_chats"
 
 DEFAULTS = {
     SERVER_URL: "",
@@ -16,6 +17,7 @@ DEFAULTS = {
     REPORT_DELETIONS: True,
     DELETED_ALPHA: 0,  # index into ALPHA_CHOICES
     CACHE_DAYS: 2,  # index into CACHE_DAYS_CHOICES
+    SHOW_REMOVED_CHATS: True,
 }
 
 # Opacity of deleted messages: well visible, marked by the trash icon.