feat: add adjustable window opacity for overlay use (#1304) (#1309)

Expose whole-window transparency via setOpacity with settings and a top-bar quick control, persisting across restarts and syncing across windows.

Co-authored-by: Cursor <cursoragent@cursor.com>

types/global/netcatty-bridge-sync.d.ts

@@ -4,6 +4,7 @@ declare global {
   interface NetcattyBridge {
     setTheme?(theme: 'light' | 'dark' | 'system'): Promise<boolean>;
     setBackgroundColor?(color: string): Promise<boolean>;
+    setWindowOpacity?(opacity: number): Promise<boolean>;
     setLanguage?(language: string): Promise<boolean>;
     // Window controls for custom title bar (Windows/Linux)
     windowMinimize?(): Promise<void>;