Save interface-scale changes from every window

Ctrl/Cmd +/-/0 in editor and connection-manager windows called
SettingsService.persist() -> saveSettings, but only the main window's RPC
served that request: it went unanswered, the new scale applied locally, and
the next broadcast from main snapped it back.

All three window RPCs now serve saveSettings through one shared
saveSettingsHandler (persist, then hand off). index.ts's settingsSaved() is
the single place settings side effects run — transfer concurrency, locale +
app menu, the preload snapshot — whichever window saved, and it rebroadcasts
to every window EXCEPT the sender: an echo arriving behind a newer local
change (a held Cmd+) would drag it backwards. Editors are skipped by their
registry key, main and the manager by name.

Known ceiling, noted at the handler: whole-object writes, so two windows
saving in the same instant means the later one wins.

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

docs/gotchas.md

@@ -210,6 +210,12 @@ touching the adjacent code.
 - Settings: the backend spread-merges (`{...DEFAULT_SETTINGS, ...data}`) so
   new keys auto-default, but the UI SettingsService reads field-by-field —
   a new setting silently stays at default until wired in all five spots.
+- **Every window's RPC must serve `saveSettings`.** The scale shortcuts
+  shipped in editor and manager windows while only the main RPC served it:
+  the request went unanswered, the change applied locally, and main's next
+  broadcast snapped it back. All three factories now share
+  `saveSettingsHandler`, and `settingsSaved()` in index.ts rebroadcasts to
+  every window but the sender — an echo would drag a held Cmd+ backwards.
 - The editor draft mirror sends full content per tick (2 MB cap upstream);
   don't shrink the 400 ms debounce.
 - `panel.touched` gates localDir auto-navigation; `connectToken` guards