feat(frontend): use Wails bindings for file I/O

Replace hidden file inputs and anchor downloads with native open/save
dialogs via a thin platform layer. WAV export sends base64 audio data to
the Go backend for writing to the user-chosen path.

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

frontend/src/lib/project.ts

@@ -1,5 +1,6 @@
 import { renderKeyboardTrack } from './audio/export';
 import type { SampleCache } from './audio/sampleCache';
+import { saveAudio, saveProject } from './platform/files';
 import type { LegacySwitchType, Marker, Project, SwitchType } from './types';
 import { app, setActiveSwitch, setMarkers, setSelectedIds } from './store.svelte';
 
@@ -32,15 +33,10 @@ export function buildProject(): Project {
   };
 }
 
-export function downloadProject(filename = 'project.json') {
+export async function downloadProject(filename = 'project.json') {
   const project = buildProject();
-  const blob = new Blob([JSON.stringify(project, null, 2)], { type: 'application/json' });
-  const url = URL.createObjectURL(blob);
-  const anchor = document.createElement('a');
-  anchor.href = url;
-  anchor.download = filename;
-  anchor.click();
-  URL.revokeObjectURL(url);
+  const content = JSON.stringify(project, null, 2);
+  await saveProject(content, filename);
 }
 
 export function loadProject(data: Project & { version?: number; switch?: string }) {
@@ -65,10 +61,5 @@ export async function exportAudio(duration: number, cache?: SampleCache) {
     cache,
   });
 
-  const url = URL.createObjectURL(blob);
-  const anchor = document.createElement('a');
-  anchor.href = url;
-  anchor.download = 'keyboard_track.wav';
-  anchor.click();
-  URL.revokeObjectURL(url);
+  await saveAudio(blob, 'keyboard_track.wav');
 }