feat(17-02): implement save flow, cover art editing, and error handling

- Wire saveEdit() to WriteTrackTagsByPath with diff-only TagChanges map
- Add cover art selection via ImageFilePicker with instant blob preview
- Add cover art removal (× button) with clearCoverArt state
- Show saving indicator and disable buttons during save
- Display errors inline in action bar; edit mode stays active on error
- Add ReadFile Go method on FrontendUtil to read cover art bytes
- Add Wails binding for ReadFile
- Clean up all edit state (editValues, pendingCoverArt, errorMessage) on close/cancel

frontend/wailsjs/go/frontendutil/FrontendUtil.d.ts

@@ -8,4 +8,6 @@ export function ImageFilePicker():Promise<string>;
 
 export function PlaylistFilePicker():Promise<Array<string>>;
 
+export function ReadFile(arg1:string):Promise<Array<number>>;
+
 export function SetContext(arg1:context.Context):Promise<void>;

frontend/wailsjs/go/frontendutil/FrontendUtil.js

@@ -14,6 +14,10 @@ export function PlaylistFilePicker() {
   return window['go']['frontendutil']['FrontendUtil']['PlaylistFilePicker']();
 }
 
+export function ReadFile(arg1) {
+  return window['go']['frontendutil']['FrontendUtil']['ReadFile'](arg1);
+}
+
 export function SetContext(arg1) {
   return window['go']['frontendutil']['FrontendUtil']['SetContext'](arg1);
 }