fix(companion): scopes, room bot backlog, DM flood fallback, atomic trail saves

- Removing a scope moves the channels that used it to the default, not to
  unscoped.
- The room bot ignores posts older than 5 minutes: the backlog a room
  replays after login no longer gets answers.
- Adverts take the default scope from the scope list, as messages do; the
  legacy fields always mirror it, and CMD_SET_DEFAULT_FLOOD_SCOPE with a
  full list answers TABLE_FULL.
- The last DM resend drops the direct path and floods.
- /trail and /waypoints are written through a temp file.
- An admin login timeout keeps the saved password.
- History records read from the SD card are sanitised.

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

examples/companion_radio/MyMesh.h

@@ -485,7 +485,7 @@ public:
   // device's own list too, not just in the two legacy fields. Empty name
   // clears the default back to list index 0 ("*"). Calls
   // rebuildRepeatScopes() and persists the list.
-  void setPrimaryScope(const char* name);
+  bool setPrimaryScope(const char* name);   // false: list full, default unchanged
 
   // Mirrors the list's current default entry back into the legacy
   // NodePrefs::default_scope_name/key pair. Those two fields are the only