fix(audio): recover clip serverFileId from audio_file_url on project load

3 fixes for audio clips losing sound after save/reload:
1. deserializeTracksList: clip-level serverFileId falls back
   to extracting from audio_file_url when server_file_id missing
2. serializeTracksList: add server_file_id to track output
   so track-level fallback survives round-trip
3. upgrade_project_json_if_needed: include server_file_id
   in source_data during legacy project upgrade

app/static/js/app.jsx

@@ -7634,6 +7634,7 @@ const serializeTracksList = (tracksList, secondsPerBar) => {
       soundfont_bank: t.soundfont_bank !== undefined ? t.soundfont_bank : (t.synth_engine ? t.synth_engine.soundfont_bank : null),
       soundfont_program: t.soundfont_program !== undefined ? t.soundfont_program : (t.synth_engine ? t.synth_engine.soundfont_program : null),
       synth_engine: t.synth_engine || undefined,
+      server_file_id: t.serverFileId || null,
       items: items
     };
   });
@@ -7648,7 +7649,7 @@ const deserializeTracksList = (schemaTracks, secondsPerBar, sectionStore) => {
     (t.items || []).forEach(item => {
       if (item.type === "AUDIO_ITEM") {
         const src = item.source_data || {};
-        const clipFileId = src.server_file_id || null;
+        const clipFileId = src.server_file_id || (src.audio_file_url ? src.audio_file_url.split('/').pop() : null) || null;
         clips.push({
           id: item.id,
           name: item.name,