feat: add advanced audio effects and improve UI

Phase 6.5 Advanced Effects:
- Add Pitch Shifter with semitones and cents adjustment
- Add Time Stretch with pitch preservation using overlap-add
- Add Distortion with soft/hard/tube types and tone control
- Add Bitcrusher with bit depth and sample rate reduction
- Add AdvancedParameterDialog with real-time waveform visualization
- Add 4 professional presets per effect type

Improvements:
- Fix undefined parameter errors by adding nullish coalescing operators
- Add global custom scrollbar styling with color-mix transparency
- Add custom-scrollbar utility class for side panel
- Improve theme-aware scrollbar appearance in light/dark modes
- Fix parameter initialization when switching effect types

Integration:
- All advanced effects support undo/redo via EffectCommand
- Effects accessible via command palette and side panel
- Selection-based processing support
- Toast notifications for all effects

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

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

app/globals.css

@@ -158,6 +158,41 @@
     @apply bg-background text-foreground;
     font-feature-settings: "rlig" 1, "calt" 1;
   }
+
+  /* Apply custom scrollbar globally */
+  * {
+    scrollbar-width: thin;
+  }
+
+  *::-webkit-scrollbar {
+    width: 10px;
+    height: 10px;
+  }
+
+  *::-webkit-scrollbar-track {
+    background: var(--muted);
+    border-radius: 5px;
+  }
+
+  *::-webkit-scrollbar-thumb {
+    background: color-mix(in oklch, var(--muted-foreground) 30%, transparent);
+    border-radius: 5px;
+    border: 2px solid var(--muted);
+    transition: background 0.2s ease;
+  }
+
+  *::-webkit-scrollbar-thumb:hover {
+    background: color-mix(in oklch, var(--muted-foreground) 50%, transparent);
+  }
+
+  *::-webkit-scrollbar-thumb:active {
+    background: color-mix(in oklch, var(--muted-foreground) 70%, transparent);
+  }
+
+  /* Scrollbar corners */
+  *::-webkit-scrollbar-corner {
+    background: var(--muted);
+  }
 }
 
 /* Custom animations */
@@ -290,22 +325,33 @@
 
 /* Custom scrollbar */
 @layer utilities {
+  .custom-scrollbar {
+    scrollbar-width: thin;
+    scrollbar-color: color-mix(in oklch, var(--muted-foreground) 30%, transparent) var(--muted);
+  }
+
   .custom-scrollbar::-webkit-scrollbar {
     width: 8px;
     height: 8px;
   }
 
   .custom-scrollbar::-webkit-scrollbar-track {
-    @apply bg-muted;
+    background: var(--muted);
     border-radius: 4px;
   }
 
   .custom-scrollbar::-webkit-scrollbar-thumb {
-    @apply bg-muted-foreground/30;
+    background: color-mix(in oklch, var(--muted-foreground) 30%, transparent);
     border-radius: 4px;
+    border: 2px solid var(--muted);
+    transition: background 0.2s ease;
   }
 
   .custom-scrollbar::-webkit-scrollbar-thumb:hover {
-    @apply bg-muted-foreground/50;
+    background: color-mix(in oklch, var(--muted-foreground) 50%, transparent);
+  }
+
+  .custom-scrollbar::-webkit-scrollbar-thumb:active {
+    background: color-mix(in oklch, var(--muted-foreground) 70%, transparent);
   }
 }