Move settings into a standalone Settings window

The tab-bar popover had outgrown itself. Settings now open in their own
native window (a fourth window kind, built like the connection manager but
not modal): a sidebar with Appearance, Transfers, Editor, File list and
About, holding every popover row plus the visible-columns choice. Each
control is an ordinary settings save, so the other windows follow live
while it stays open.

The gear button in the tab bar, Ctrl+, and the macOS app menu's
"Settings..." (Cmd+,) open it. On macOS Cmd+W arrives as the main window's
close-tab menu action, so bun closes the focused Settings window first
instead of a main-window tab.

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

ui/src/app/skins/shadlike/app.ts

@@ -188,6 +188,9 @@ export class App {
         // ⌘1–⌘8 jump to that tab, ⌘9 to the last, like Firefox
         this.panels.goToWorkspace(match.digit === 9 ? -1 : match.digit! - 1);
         break;
+      case 'open-settings':
+        void this.rpc.request.openSettings({});
+        break;
       case 'scale-up':
         this.settings.setUiScale(this.settings.uiScale() + 0.1);
         break;