feat(v3): add desktop-only "Start in fullscreen" system option

Adds a "Start in fullscreen" toggle to the Settings → System panel,
addressing the desktop request in feedBack-desktop#97: users want the
app to launch fullscreen without hitting the OS hotkey every time.

The block ships hidden and is gated exactly like the App-updates block:
setupWindowOptions() only unhides + wires it when the feedBack-desktop
bridge exposes window.feedBackDesktop.window.{getStartFullscreen,
setStartFullscreen}. Web/Docker builds have no such bridge, so the
section never appears there. Persistence lives desktop-side because
only the Electron main process can read the pref at window-creation
time — core just proxies through the bridge.

The desktop bridge + launch behaviour land in a follow-up
feedBack-desktop PR.

Signed-off-by: gionnibgud <gionnibgud@gmail.com>

static/app.js

@@ -218,6 +218,7 @@ import {
     setAvOffsetMs,
     setInstrumentPathway,
     setupAppUpdates,
+    setupWindowOptions,
     syncDefaultArrangementPin,
 } from './js/settings.js';
 import {