refactor: remove master effects section, add TODO note

Removed the master effects section from AudioEditor footer:
- Simplified footer to only show transport controls
- Removed master channel strip area
- Removed border separator between sections

Added note to PLAN.md:
- 🔲 Master channel effects (TODO)
- Will implement master effect chain UI later
- Similar to per-track effects but for final mix
- Documented in Phase 7 multi-track features section

This cleans up the UI for now - we'll add master effects
with a proper device rack UI later, similar to how per-track
effects work (collapsible section with horizontal device cards).

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

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

PLAN.md

@@ -94,6 +94,12 @@
 - ✅ Synchronized playback across all tracks
 - ✅ Per-track gain and pan during playback
 - ✅ Solo/Mute handling during playback
+- ✅ Per-track effect chains with device rack
+- ✅ Collapsible effects section below each track
+- ✅ Effect browser with categorized effects
+- ✅ Horizontal scrolling device rack (Ableton-style)
+- ✅ Individual effect cards with expand/collapse
+- 🔲 Master channel effects (TODO: Implement master effect chain UI similar to per-track effects)
 
 ### Next Steps
 - **Phase 6**: Audio effects ✅ COMPLETE (Basic + Filters + Dynamics + Time-Based + Advanced + Chain Management)