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/architecture.md

@@ -218,10 +218,13 @@ component; tokens → `ui/src/styles.css`.
   skip, overwrite dialog with all/none batch modes).
 - `settings.service.ts` — one signal per setting; **imperative apply pattern**
   (`applyTheme/DarkTone/Accent` set `data-*` attributes on `<html>`;
-  `applyUiScale` sets its inline `font-size`); `applyAll` also serves the `settingsChanged` broadcast so editor
-  and manager windows restyle live (they render no settings UI, so no
-  persist echo). New setting = five touches here + types.ts (backend merges
-  automatically).
+  `applyUiScale` sets its inline `font-size`); `applyAll` also serves the
+  `settingsChanged` broadcast, so every window follows a save made in any
+  other. All three window RPCs serve `saveSettings` (editor and manager windows
+  save too — the scale shortcuts); bun's `settingsSaved()` in index.ts runs the
+  side effects once and rebroadcasts to every window **except the sender**, so
+  there is no echo to fight a held Cmd+. `applyAll` never persists. New setting
+  = five touches here + types.ts (backend merges automatically).
 - `focus.service.ts` — dependency-free `tick`/`bump()`; dialogs bump it, the
   main window's panels listen. Exists so dialogs don't inject PanelsService
   (whose constructor spawns a workspace — fatal in secondary windows).