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>

components/ui/Button.tsx

@@ -4,7 +4,7 @@ import { cn } from '@/lib/utils/cn';
 export interface ButtonProps
   extends React.ButtonHTMLAttributes<HTMLButtonElement> {
   variant?: 'default' | 'destructive' | 'outline' | 'secondary' | 'ghost' | 'link';
-  size?: 'default' | 'sm' | 'lg' | 'icon';
+  size?: 'default' | 'sm' | 'lg' | 'icon' | 'icon-sm';
 }
 
 const Button = React.forwardRef<HTMLButtonElement, ButtonProps>(
@@ -32,6 +32,7 @@ const Button = React.forwardRef<HTMLButtonElement, ButtonProps>(
             'h-9 rounded-md px-3': size === 'sm',
             'h-11 rounded-md px-8': size === 'lg',
             'h-10 w-10': size === 'icon',
+            'h-8 w-8': size === 'icon-sm',
           },
           className
         )}