Make ProjectFile struct support both v1 and v2 formats

audio_file, source_file, audio_wav are all optional with serde defaults.
v1 projects have audio_file, v2 projects have source_file + audio_wav.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src-tauri/src/commands/project.rs

@@ -12,7 +12,12 @@ use crate::state::AppState;
 pub struct ProjectFile {
     pub version: u32,
     pub name: String,
-    pub audio_file: String,
+    #[serde(default)]
+    pub audio_file: Option<String>,
+    #[serde(default)]
+    pub source_file: Option<String>,
+    #[serde(default)]
+    pub audio_wav: Option<String>,
     pub created_at: String,
     pub segments: Vec<ProjectFileSegment>,
     pub speakers: Vec<ProjectFileSpeaker>,