refactor: make track control panel full height and remove redundant buttons

Changes:
- Track control panel now uses flexbox (full height, no scrolling)
- Removed "Add Empty Track" and "Import Audio Files" buttons from main area
- All track management is now done via the sidebar only
- Cleaner, more professional DAW-style interface

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/tracks/Track.tsx

@@ -171,7 +171,7 @@ export function Track({
     >
       {/* Left: Track Control Panel (Fixed Width) */}
       <div
-        className="w-72 flex-shrink-0 bg-card border-r border-border p-3 space-y-2 overflow-y-auto"
+        className="w-72 flex-shrink-0 bg-card border-r border-border p-3 flex flex-col gap-2"
         onClick={(e) => e.stopPropagation()}
       >
         {/* Track Name & Collapse Toggle */}