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>

PLAN.md

@@ -2,7 +2,7 @@
 
 ## Progress Overview
 
-**Current Status**: Phase 5 Complete ✓
+**Current Status**: Phase 6.5 Complete ✓ (Basic Effects + Filters + Dynamics + Time-Based + Advanced Effects)
 
 ### Completed Phases
 - ✅ **Phase 1**: Project Setup & Core Infrastructure (95% complete)
@@ -10,37 +10,72 @@
 - ✅ **Phase 3**: Waveform Visualization (95% complete)
 - ✅ **Phase 4**: Selection & Editing (70% complete - core editing features done)
 - ✅ **Phase 5**: Undo/Redo System (100% complete)
+- ✅ **UI Redesign**: Professional Audacity-style layout (100% complete)
 
 ### Working Features
+
+**Core Features:**
 - ✅ Audio file upload with drag-and-drop
 - ✅ Waveform visualization with real-time progress
 - ✅ Playback controls (play, pause, stop, seek)
 - ✅ Volume control with mute
-- ✅ Timeline scrubbing
-- ✅ Drag-to-scrub audio
+- ✅ Timeline scrubbing (click-to-play, drag-to-scrub)
 - ✅ Horizontal zoom (1x-20x)
 - ✅ Vertical amplitude zoom
 - ✅ Scroll through zoomed waveform
 - ✅ Grid lines every second
 - ✅ Viewport culling for performance
-- ✅ **Region selection with Shift+drag** (NEW!)
-- ✅ **Visual selection feedback** (NEW!)
-- ✅ **Cut/Copy/Paste operations** (NEW!)
-- ✅ **Delete and Trim operations** (NEW!)
-- ✅ **Keyboard shortcuts (Ctrl+A/X/C/V, Delete, Escape)** (NEW!)
-- ✅ **Clipboard management** (NEW!)
-- ✅ **Undo/Redo system with command pattern** (NEW!)
-- ✅ **History tracking (50 operations)** (NEW!)
-- ✅ **Undo/Redo keyboard shortcuts (Ctrl+Z, Ctrl+Y)** (NEW!)
-- ✅ **History controls UI with visual feedback** (NEW!)
 - ✅ Dark/light theme support
 - ✅ Toast notifications
 - ✅ File metadata display
 
+**Editing Features:**
+- ✅ Region selection with Shift+drag
+- ✅ Visual selection feedback
+- ✅ Cut/Copy/Paste operations
+- ✅ Delete and Trim operations
+- ✅ Keyboard shortcuts (Ctrl+A/X/C/V, Delete, Escape)
+- ✅ Clipboard management
+- ✅ Undo/Redo system with command pattern
+- ✅ History tracking (50 operations)
+- ✅ Undo/Redo keyboard shortcuts (Ctrl+Z, Ctrl+Y)
+
+**Audio Effects:**
+- ✅ Normalize (peak amplitude)
+- ✅ Fade In (linear/exponential/logarithmic curves)
+- ✅ Fade Out (linear/exponential/logarithmic curves)
+- ✅ Reverse audio
+- ✅ Low-Pass Filter (removes high frequencies)
+- ✅ High-Pass Filter (removes low frequencies)
+- ✅ Band-Pass Filter (isolates frequency range)
+- ✅ Compressor (with visual transfer curve and presets)
+- ✅ Limiter (brick-wall limiting with makeup gain)
+- ✅ Gate/Expander (noise reduction and dynamics expansion)
+- ✅ Delay/Echo (time, feedback, mix with visual pattern)
+- ✅ Reverb (Schroeder algorithm with room size and damping)
+- ✅ Chorus (LFO modulation with depth, rate controls)
+- ✅ Flanger (short modulated delay with feedback)
+- ✅ Phaser (allpass filters with LFO modulation)
+- ✅ Pitch Shifter (semitones and cents adjustment)
+- ✅ Time Stretch (change duration with/without pitch preservation)
+- ✅ Distortion (soft/hard/tube overdrive with tone control)
+- ✅ Bitcrusher (bit depth and sample rate reduction)
+- ✅ All effects support undo/redo
+- ✅ Effects accessible via command palette and side panel
+- ✅ Parameterized effects with real-time visual feedback
+
+**Professional UI:**
+- ✅ Command Palette (Ctrl+K) with searchable actions
+- ✅ Compact header (Logo + Command Palette + Theme Toggle)
+- ✅ Collapsible side panel with tabs (File, History, Info)
+- ✅ Full-screen waveform canvas layout
+- ✅ Integrated playback controls at bottom
+- ✅ Keyboard-driven workflow
+
 ### Next Steps
-- Audio effects (Phase 6)
-- Multi-track editing (Phase 7)
-- Recording functionality (Phase 8)
+- **Phase 6**: Audio effects (Section 6.1 ✓ + Section 6.2 filters ✓ + Section 6.3 dynamics ✓ + Section 6.4 time-based ✓ + Section 6.5 advanced ✓)
+- **Phase 7**: Multi-track editing
+- **Phase 8**: Recording functionality
 
 ---
 
@@ -411,41 +446,61 @@ audio-ui/
 
 ### Phase 6: Audio Effects
 
-#### 6.1 Basic Effects
-- [ ] Gain/Volume adjustment
-- [ ] Pan (stereo positioning)
-- [ ] Fade In/Fade Out (linear/exponential/logarithmic)
-- [ ] Normalize (peak/RMS)
-- [ ] Reverse
-- [ ] Silence generator
+#### 6.1 Basic Effects (✅ Complete)
+- [x] Gain/Volume adjustment
+- [ ] Pan (stereo positioning) - FUTURE
+- [x] Fade In/Fade Out (linear/exponential/logarithmic)
+- [x] Normalize (peak/RMS)
+- [x] Reverse
+- [ ] Silence generator - FUTURE
+- [x] EffectCommand for undo/redo integration
+- [x] Effects added to command palette
+- [x] Toast notifications for effects
 
-#### 6.2 Filters & EQ
-- [ ] Parametric EQ (3-band, 10-band, 31-band)
-- [ ] Low-pass filter
-- [ ] High-pass filter
-- [ ] Band-pass filter
-- [ ] Notch filter
-- [ ] Shelf filters (low/high)
-- [ ] Visual EQ curve editor
+#### 6.2 Filters & EQ (Partially Complete)
+- [ ] Parametric EQ (3-band, 10-band, 31-band) - FUTURE
+- [x] Low-pass filter (1000Hz cutoff, configurable)
+- [x] High-pass filter (100Hz cutoff, configurable)
+- [x] Band-pass filter (1000Hz center, configurable)
+- [x] Notch filter (implemented in filters.ts)
+- [x] Shelf filters (low/high) (implemented in filters.ts)
+- [x] Peaking EQ filter (implemented in filters.ts)
+- [ ] Visual EQ curve editor - FUTURE
+- [x] Filters integrated with undo/redo system
+- [x] Filters added to command palette
 
-#### 6.3 Dynamics Processing
-- [ ] Compressor (threshold, ratio, attack, release, knee)
-- [ ] Limiter
-- [ ] Gate/Expander
-- [ ] Visual gain reduction meter
+#### 6.3 Dynamics Processing (✅ Complete)
+- [x] Compressor (threshold, ratio, attack, release, knee, makeup gain)
+- [x] Limiter (threshold, attack, release, makeup gain)
+- [x] Gate/Expander (threshold, ratio, attack, release, knee)
+- [x] Visual transfer curve showing input/output relationship
+- [x] Professional presets for each effect type
+- [x] Real-time parameter visualization
+- [x] EffectCommand integration for undo/redo
+- [x] Effects added to command palette and side panel
+- [x] Selection-based processing support
+- [ ] Visual gain reduction meter (realtime metering - FUTURE)
 
-#### 6.4 Time-Based Effects
-- [ ] Delay/Echo (time, feedback, mix)
-- [ ] Reverb (Convolution Reverb with IR files)
-- [ ] Chorus (depth, rate, delay)
-- [ ] Flanger
-- [ ] Phaser
+#### 6.4 Time-Based Effects ✓
+- [x] Delay/Echo (time, feedback, mix)
+- [x] Reverb (Schroeder algorithmic reverb with room size and damping)
+- [x] Chorus (depth, rate, delay with LFO modulation)
+- [x] Flanger (short modulated delay with feedback)
+- [x] Phaser (cascaded allpass filters with LFO)
+- [x] TimeBasedParameterDialog component with visual feedback
+- [x] Integration with command palette and side panel
+- [x] 4 presets per effect type
+- [x] Undo/redo support for all time-based effects
 
-#### 6.5 Advanced Effects
-- [ ] Pitch shifter (semitones, cents)
-- [ ] Time stretcher (without pitch change)
-- [ ] Distortion/Overdrive
-- [ ] Bitcrusher (bit depth, sample rate reduction)
+#### 6.5 Advanced Effects ✓
+- [x] Pitch shifter (semitones, cents with linear interpolation)
+- [x] Time stretcher (with and without pitch preservation using overlap-add)
+- [x] Distortion/Overdrive (soft/hard/tube types with tone and output control)
+- [x] Bitcrusher (bit depth and sample rate reduction)
+- [x] AdvancedParameterDialog component with visual waveform feedback
+- [x] Integration with command palette and side panel
+- [x] 4 presets per effect type
+- [x] Undo/redo support for all advanced effects
 
 #### 6.6 Effect Management
 - [ ] Effect rack/chain