fix(05-jamtrack): implement pickMyPackage logic for mixdown selection

The downloadJamTrack thunk was failing with "No mixdown package available"
because it was trying to access mixdowns[0].packageId directly. Mixdowns
have a packages array, and the correct package must be selected based on
client capabilities (pickMyPackage logic from legacy implementation).

Changes to downloadJamTrack thunk:
- Get client sample rate via jamClient.GetSampleRate()
- Find mixdown by mixdownId if provided, else use first mixdown
- Implement pickMyPackage logic to find compatible package:
  * file_type === 'ogg'
  * encrypt_type === 'jkz'
  * sample_rate === clientSampleRate (44 or 48 kHz)
- Extract packageId from selected package
- Add detailed error messages for each failure case

This matches the legacy JamTrackStore.pickMyPackage() behavior documented
in .planning/codebase/JAMTRACK_LEGACY.md lines 126-130.

Fixes "No mixdown package available" error during JamTrack playback.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

jam-ui/src/store/features/mediaSlice.js

@@ -59,15 +59,38 @@ export const downloadJamTrack = createAsyncThunk(
         progress: 0
       }));
 
-      // Get package ID (simplified - use first available package)
-      // Real implementation would use pickMyPackage() logic from legacy
-      const mixdowns = await jamClient.JamTrackGetMixdowns(jamTrack.id);
-      const packageId = mixdowns && mixdowns.length > 0 ? mixdowns[0].packageId : null;
+      // Get client sample rate for package selection (pickMyPackage logic)
+      const sampleRate = await jamClient.GetSampleRate();
 
-      if (!packageId) {
-        throw new Error('No mixdown package available');
+      // Get mixdowns and find compatible package
+      const mixdowns = await jamClient.JamTrackGetMixdowns(jamTrack.id);
+
+      if (!mixdowns || mixdowns.length === 0) {
+        throw new Error('No mixdowns available for this JamTrack');
       }
 
+      // Find the first mixdown (use selected mixdownId if provided, else use first)
+      const mixdown = mixdownId
+        ? mixdowns.find(m => m.id === mixdownId) || mixdowns[0]
+        : mixdowns[0];
+
+      if (!mixdown || !mixdown.packages || mixdown.packages.length === 0) {
+        throw new Error('Mixdown has no packages available');
+      }
+
+      // pickMyPackage logic: find compatible package (ogg, jkz, matching sample rate)
+      const compatiblePackage = mixdown.packages.find(pkg =>
+        pkg.file_type === 'ogg' &&
+        pkg.encrypt_type === 'jkz' &&
+        pkg.sample_rate === sampleRate
+      );
+
+      if (!compatiblePackage) {
+        throw new Error(`No compatible package found for sample rate ${sampleRate}kHz`);
+      }
+
+      const packageId = compatiblePackage.id;
+
       // Set up global callbacks for native client
       window.jamTrackDownloadProgress = (percent) => {
         dispatch(setDownloadState({ progress: percent }));