fix(audio): robust async buffer loading with state merging + API fallback

- Rewrite loadAudioBuffersForTracks to use functional setTracks
  preventing loss of intermediate state updates
- Add API download endpoint as fallback when static fetch fails
- Also update sessionTabs tracks with loaded buffers
- return decodeAudioFile result (audioBuffer+channelInfo) instead
  of just audioBuffer

app/static/js/app.jsx

@@ -9860,65 +9860,81 @@ const App = () => {
 
   const loadAudioBuffersForTracks = async (tracksList) => {
     let hasLoadedAny = false;
+    const loadBuffer = async (url) => {
+      const res = await fetch(url);
+      if (!res.ok) return null;
+      const blob = await res.blob();
+      return await window.SonicAudio.decodeAudioFile(blob);
+    };
+    const tryLoad = async (fileId) => {
+      if (!fileId) return null;
+      try {
+        const result = await loadBuffer('/static/audio/uploads/' + fileId);
+        if (result) return result;
+      } catch (_) {}
+      try {
+        const result = await loadBuffer(`${API_AUDIO}/download/${fileId}`);
+        if (result) return result;
+      } catch (_) {}
+      return null;
+    };
     const updatedTracks = await Promise.all(tracksList.map(async t => {
       let trackBuffer = t.buffer;
       let trackChannelInfo = t.channelInfo;
       if (t.serverFileId && !trackBuffer) {
-        try {
-          const res = await fetch('/static/audio/uploads/' + t.serverFileId);
-          if (res.ok) {
-            const blob = await res.blob();
-            const { audioBuffer, channelInfo } = await window.SonicAudio.decodeAudioFile(blob);
-            trackBuffer = audioBuffer;
-            trackChannelInfo = channelInfo;
-            hasLoadedAny = true;
-          }
-        } catch (e) {
-          console.warn("Failed to autoload track buffer for " + t.serverFileId, e);
+        const result = await tryLoad(t.serverFileId);
+        if (result) {
+          trackBuffer = result.audioBuffer;
+          trackChannelInfo = result.channelInfo;
+          hasLoadedAny = true;
         }
       }
-      
-      let clipsUpdated = false;
       const updatedClips = await Promise.all((t.clips || []).map(async c => {
         let clipBuffer = c.buffer;
         const targetFileId = c.serverFileId || t.serverFileId;
         if (targetFileId && !clipBuffer) {
-          try {
-            const res = await fetch('/static/audio/uploads/' + targetFileId);
-            if (res.ok) {
-              const blob = await res.blob();
-              const { audioBuffer } = await window.SonicAudio.decodeAudioFile(blob);
-              clipBuffer = audioBuffer;
-              clipsUpdated = true;
-              hasLoadedAny = true;
-            }
-          } catch (e) {
-            console.warn("Failed to autoload clip buffer for " + targetFileId, e);
+          const result = await tryLoad(targetFileId);
+          if (result) {
+            clipBuffer = result.audioBuffer;
+            hasLoadedAny = true;
           }
         }
         return { ...c, buffer: clipBuffer };
       }));
-
       if (trackBuffer && updatedClips.length === 0) {
         const clipId = `default_${t.id}`;
-        return {
-          ...t,
-          buffer: trackBuffer,
-          channelInfo: trackChannelInfo,
-          clips: [{ id: clipId, buffer: trackBuffer, startTime: t.startTime || 0, name: t.name, speed: 1.0 }]
-        };
+        return { ...t, buffer: trackBuffer, channelInfo: trackChannelInfo,
+          clips: [{ id: clipId, buffer: trackBuffer, startTime: t.startTime || 0, name: t.name, speed: 1.0 }] };
       }
-
-      return {
-        ...t,
-        buffer: trackBuffer,
-        channelInfo: trackChannelInfo,
-        clips: updatedClips
-      };
+      return { ...t, buffer: trackBuffer, channelInfo: trackChannelInfo, clips: updatedClips };
     }));
-
     if (hasLoadedAny) {
-      setTracks(updatedTracks);
+      setTracks(prev => {
+        const merged = [...updatedTracks];
+        (prev || []).forEach((pt, i) => {
+          if (!merged[i]) merged[i] = pt;
+          else {
+            merged[i] = { ...merged[i] };
+            merged[i].clips = (pt.clips || []).map((pc, j) => {
+              if (merged[i].clips && merged[i].clips[j] && (merged[i].clips[j].buffer || pc.buffer)) {
+                return { ...pc, buffer: pc.buffer || merged[i].clips[j].buffer };
+              }
+              if ((pc.buffer || (merged[i].clips && merged[i].clips[j] && merged[i].clips[j].buffer))) {
+                return pc;
+              }
+              return merged[i].clips && merged[i].clips[j] ? merged[i].clips[j] : pc;
+            });
+          }
+        });
+        return merged;
+      });
+      setSessionTabs(prev => prev.map(st => ({
+        ...st,
+        tracks: (st.tracks || []).map(t => {
+          const found = updatedTracks.find(u => u.id === t.id);
+          return found || t;
+        })
+      })));
     }
   };