feat(09-03): add scan control UI with pause/resume/cancel and confirmation dialog

- Pause/Resume buttons replace Soft Scan/Full Rescan during active scan
- Cancel button shows confirmation dialog with Keep/Discard/Continue options
- Register LibraryScanPaused/Resumed/Cancelled event handlers
- Status bar shows 'Scan paused.' with accent color when paused
- Add CancelScan/PauseScan/ResumeScan Wails binding stubs
- Cancel dialog overlay dismissible by clicking outside or Continue Scanning
- Add cancelled field to ScanMetrics TypeScript interface

frontend/wailsjs/go/library/Library.d.ts

@@ -3,10 +3,16 @@
 import {library} from '../models';
 import {context} from '../models';
 
+export function CancelScan():Promise<void>;
+
 export function FullRescan():Promise<library.ScanMetrics>;
 
 export function GetAlbumTracks(arg1:number):Promise<Array<library.Track>>;
 
+export function IsScanActive():Promise<boolean>;
+
+export function IsScanPaused():Promise<boolean>;
+
 export function GetAlbumsByArtist(arg1:number):Promise<Array<library.Album>>;
 
 export function GetAllAlbums():Promise<Array<library.Album>>;
@@ -19,6 +25,10 @@ export function GetAllTracks():Promise<Array<library.Track>>;
 
 export function GetTracksByGenre(arg1:string):Promise<Array<library.Track>>;
 
+export function PauseScan():Promise<void>;
+
+export function ResumeScan():Promise<void>;
+
 export function Scan():Promise<library.ScanMetrics>;
 
 export function SearchTracks(arg1:string):Promise<Array<library.Track>>;