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>

fileservice.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"encoding/base64"
 	"net/http"
 	"os"
 	"path/filepath"
@@ -102,11 +103,16 @@ func (s *FileService) SaveProject(content string, defaultName string) error {
 	return os.WriteFile(path, []byte(content), 0o644)
 }
 
-func (s *FileService) SaveAudio(data []byte, defaultName string) error {
+func (s *FileService) SaveAudio(dataBase64 string, defaultName string) error {
 	if defaultName == "" {
 		defaultName = "keyboard_track.wav"
 	}
 
+	data, err := base64.StdEncoding.DecodeString(dataBase64)
+	if err != nil {
+		return err
+	}
+
 	path, err := s.app.Dialog.SaveFile().
 		SetMessage("Export Audio").
 		SetFilename(defaultName).