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/core/context-menu.service.ts

@@ -5,7 +5,7 @@ import { IS_MAC, IS_WINDOWS, MOD_LABEL } from './platform';
 import { RpcService } from './rpc.service';
 
 /** Optional list columns, in display order (Name is fixed). */
-const COLUMN_KEYS: ColumnKey[] = ['size', 'mtime', 'kind', 'rights', 'owner', 'group'];
+export const COLUMN_KEYS: ColumnKey[] = ['size', 'mtime', 'kind', 'rights', 'owner', 'group'];
 
 export interface MenuItem {
   type?: 'separator';