fix: render instrument description in tooltip instead of object

Error: "Objects are not valid as a React child (found: object with keys
{id, description, created_at, updated_at, popularity})"

Root cause:
- JKSessionMyTrack.js line 191 was rendering {track?.instrument} directly
- track.instrument is an object, not a string
- React cannot render objects as children - they must be primitives

This error appeared when JamTrack stems were displayed on the session
screen after selecting a JamTrack. The stems use JKSessionMyTrack
components which have instrument tooltips.

Solution:
Changed tooltip content from:
  {track?.instrument}
To:
  {track?.instrument?.description || track?.instrument?.name || 'Unknown Instrument'}

This extracts the description string from the instrument object, with
fallbacks to name or a default string.

Fixes crash when hovering over JamTrack stem instruments.

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

jam-ui/src/components/client/JKSessionMyTrack.js

@@ -188,7 +188,7 @@ const JKSessionMyTrack = ({
             target={`instrument-tooltip-${clientId}-${track?.client_track_id || 'chat'}`}
             trigger="hover click"
           >
-            {track?.instrument}
+            {track?.instrument?.description || track?.instrument?.name || 'Unknown Instrument'}
           </UncontrolledTooltip>
         )}
         <div className="track-controls">