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/package.json

@@ -13,6 +13,7 @@
     "@sveltejs/vite-plugin-svelte": "^7.1.2",
     "@tsconfig/svelte": "^5.0.8",
     "@types/node": "^24.12.3",
+    "@wailsio/runtime": "^3.0.0-alpha.95",
     "svelte": "^5.55.5",
     "svelte-check": "^4.4.8",
     "typescript": "~6.0.2",