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>

components/editor/AudioEditor.tsx

@@ -328,37 +328,22 @@ export function AudioEditor() {
         </main>
       </div>
 
-      {/* Transport Controls + Master Channel */}
-      <div className="border-t border-border bg-card">
-        {/* Transport Controls */}
-        <div className="p-3 flex justify-center border-b border-border">
-          <PlaybackControls
-            isPlaying={isPlaying}
-            isPaused={!isPlaying}
-            currentTime={currentTime}
-            duration={duration}
-            volume={masterVolume}
-            onPlay={play}
-            onPause={pause}
-            onStop={stop}
-            onSeek={seek}
-            onVolumeChange={setMasterVolume}
-            currentTimeFormatted={formatDuration(currentTime)}
-            durationFormatted={formatDuration(duration)}
-          />
-        </div>
-
-        {/* Master Track Strip */}
-        <div className="p-3">
-          <div className="flex items-center gap-2 mb-2">
-            <h3 className="text-sm font-semibold text-foreground">Master</h3>
-            <span className="text-xs text-muted-foreground">Final mix output</span>
-          </div>
-          {/* Master effects will go here */}
-          <div className="text-xs text-muted-foreground">
-            Master effects: {masterEffectChain.effects.length}
-          </div>
-        </div>
+      {/* Transport Controls */}
+      <div className="border-t border-border bg-card p-3 flex justify-center">
+        <PlaybackControls
+          isPlaying={isPlaying}
+          isPaused={!isPlaying}
+          currentTime={currentTime}
+          duration={duration}
+          volume={masterVolume}
+          onPlay={play}
+          onPause={pause}
+          onStop={stop}
+          onSeek={seek}
+          onVolumeChange={setMasterVolume}
+          currentTimeFormatted={formatDuration(currentTime)}
+          durationFormatted={formatDuration(duration)}
+        />
       </div>
 
       {/* Import Track Dialog */}