feat: add Phase 6.6 Effect Chain Management system

Effect Chain System:
- Create comprehensive effect chain types and state management
- Implement EffectRack component with drag-and-drop reordering
- Add enable/disable toggle for individual effects
- Build PresetManager for save/load/import/export functionality
- Create useEffectChain hook with localStorage persistence

UI Integration:
- Add Chain tab to SidePanel with effect rack
- Integrate preset manager dialog
- Add chain management controls (clear, presets)
- Update SidePanel with chain props and handlers

Features:
- Drag-and-drop effect reordering with visual feedback
- Effect bypass/enable toggle with power icons
- Save effect chains as presets with descriptions
- Import/export presets as JSON files
- localStorage persistence for chains and presets
- Visual status indicators for enabled/disabled effects
- Preset timestamp and effect count display

Components Created:
- /lib/audio/effects/chain.ts - Effect chain types and utilities
- /components/effects/EffectRack.tsx - Visual effect chain component
- /components/effects/PresetManager.tsx - Preset management dialog
- /lib/hooks/useEffectChain.ts - Effect chain state hook

Updated PLAN.md:
- Mark Phase 6.6 as complete
- Update current status to Phase 6.6 Complete
- Add effect chain features to working features list
- Update Next Steps to show Phase 6 complete

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

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

components/layout/SidePanel.tsx

@@ -11,12 +11,18 @@ import {
   Download,
   X,
   Sparkles,
+  Link2,
+  FolderOpen,
+  Trash2,
 } from 'lucide-react';
 import { Button } from '@/components/ui/Button';
 import { cn } from '@/lib/utils/cn';
 import { formatDuration } from '@/lib/audio/decoder';
 import type { Selection } from '@/types/selection';
 import type { HistoryState } from '@/lib/history/history-manager';
+import type { EffectChain, ChainEffect, EffectPreset } from '@/lib/audio/effects/chain';
+import { EffectRack } from '@/components/effects/EffectRack';
+import { PresetManager } from '@/components/effects/PresetManager';
 
 export interface SidePanelProps {
   // File info
@@ -31,6 +37,17 @@ export interface SidePanelProps {
   // History info
   historyState: HistoryState;
 
+  // Effect chain
+  effectChain: EffectChain;
+  effectPresets: EffectPreset[];
+  onToggleEffect: (effectId: string) => void;
+  onRemoveEffect: (effectId: string) => void;
+  onReorderEffects: (fromIndex: number, toIndex: number) => void;
+  onSavePreset: (preset: EffectPreset) => void;
+  onLoadPreset: (preset: EffectPreset) => void;
+  onDeletePreset: (presetId: string) => void;
+  onClearChain: () => void;
+
   // Effects handlers
   onNormalize: () => void;
   onFadeIn: () => void;
@@ -62,6 +79,15 @@ export function SidePanel({
   onClear,
   selection,
   historyState,
+  effectChain,
+  effectPresets,
+  onToggleEffect,
+  onRemoveEffect,
+  onReorderEffects,
+  onSavePreset,
+  onLoadPreset,
+  onDeletePreset,
+  onClearChain,
   onNormalize,
   onFadeIn,
   onFadeOut,
@@ -84,7 +110,8 @@ export function SidePanel({
   className,
 }: SidePanelProps) {
   const [isCollapsed, setIsCollapsed] = React.useState(false);
-  const [activeTab, setActiveTab] = React.useState<'file' | 'history' | 'info' | 'effects'>('file');
+  const [activeTab, setActiveTab] = React.useState<'file' | 'chain' | 'history' | 'info' | 'effects'>('file');
+  const [presetDialogOpen, setPresetDialogOpen] = React.useState(false);
   const fileInputRef = React.useRef<HTMLInputElement>(null);
 
   const handleFileClick = () => {
@@ -131,6 +158,14 @@ export function SidePanel({
           >
             <FileAudio className="h-4 w-4" />
           </Button>
+          <Button
+            variant={activeTab === 'chain' ? 'secondary' : 'ghost'}
+            size="icon-sm"
+            onClick={() => setActiveTab('chain')}
+            title="Effect Chain"
+          >
+            <Link2 className="h-4 w-4" />
+          </Button>
           <Button
             variant={activeTab === 'effects' ? 'secondary' : 'ghost'}
             size="icon-sm"
@@ -227,6 +262,53 @@ export function SidePanel({
           </>
         )}
 
+        {activeTab === 'chain' && (
+          <div className="space-y-2">
+            <div className="flex items-center justify-between">
+              <h3 className="text-xs font-semibold text-muted-foreground uppercase">
+                Effect Chain
+              </h3>
+              <div className="flex gap-1">
+                <Button
+                  variant="ghost"
+                  size="icon-sm"
+                  onClick={() => setPresetDialogOpen(true)}
+                  title="Manage presets"
+                >
+                  <FolderOpen className="h-4 w-4" />
+                </Button>
+                {effectChain.effects.length > 0 && (
+                  <Button
+                    variant="ghost"
+                    size="icon-sm"
+                    onClick={onClearChain}
+                    title="Clear all effects"
+                  >
+                    <Trash2 className="h-4 w-4 text-destructive" />
+                  </Button>
+                )}
+              </div>
+            </div>
+            <EffectRack
+              chain={effectChain}
+              onToggleEffect={onToggleEffect}
+              onRemoveEffect={onRemoveEffect}
+              onReorderEffects={onReorderEffects}
+            />
+            <PresetManager
+              open={presetDialogOpen}
+              onClose={() => setPresetDialogOpen(false)}
+              currentChain={effectChain}
+              presets={effectPresets}
+              onSavePreset={onSavePreset}
+              onLoadPreset={onLoadPreset}
+              onDeletePreset={onDeletePreset}
+              onExportPreset={() => {}}
+              onImportPreset={(preset) => onSavePreset(preset)}
+            />
+          </div>
+        )}
+
         {activeTab === 'history' && (
           <div className="space-y-2">
             <h3 className="text-xs font-semibold text-muted-foreground uppercase">