feat: implement Phase 5 - undo/redo system with command pattern

Added comprehensive undo/redo functionality:
- Command pattern interface and base classes
- HistoryManager with 50-operation stack
- EditCommand for all edit operations (cut, delete, paste, trim)
- Full keyboard shortcuts (Ctrl+Z undo, Ctrl+Y/Ctrl+Shift+Z redo)
- HistoryControls UI component with visual feedback
- Integrated history system with all edit operations
- Toast notifications for undo/redo actions
- History state tracking and display

New files:
- lib/history/command.ts - Command interface and BaseCommand
- lib/history/history-manager.ts - HistoryManager class
- lib/history/commands/edit-command.ts - EditCommand and factory functions
- lib/hooks/useHistory.ts - React hook for history management
- components/editor/HistoryControls.tsx - History UI component

Modified files:
- components/editor/AudioEditor.tsx - Integrated history system
- components/editor/EditControls.tsx - Updated keyboard shortcuts display

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

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

components/editor/HistoryControls.tsx

@@ -0,0 +1,107 @@
+'use client';
+
+import * as React from 'react';
+import { Undo2, Redo2, History, Info } from 'lucide-react';
+import { Button } from '@/components/ui/Button';
+import { cn } from '@/lib/utils/cn';
+import type { HistoryState } from '@/lib/history/history-manager';
+
+export interface HistoryControlsProps {
+  historyState: HistoryState;
+  onUndo: () => void;
+  onRedo: () => void;
+  onClear?: () => void;
+  className?: string;
+}
+
+export function HistoryControls({
+  historyState,
+  onUndo,
+  onRedo,
+  onClear,
+  className,
+}: HistoryControlsProps) {
+  return (
+    <div className={cn('space-y-4', className)}>
+      {/* History Info */}
+      {historyState.historySize > 0 && (
+        <div className="rounded-lg border border-info bg-info/10 p-3">
+          <div className="flex items-start gap-2">
+            <Info className="h-4 w-4 text-info-foreground mt-0.5 flex-shrink-0" />
+            <div className="flex-1 min-w-0 text-sm">
+              <p className="font-medium text-info-foreground">History Available</p>
+              <p className="text-info-foreground/90 mt-1">
+                {historyState.historySize} action{historyState.historySize !== 1 ? 's' : ''} in history
+              </p>
+              {historyState.undoDescription && (
+                <p className="text-xs text-info-foreground/75 mt-1">
+                  Next undo: {historyState.undoDescription}
+                </p>
+              )}
+              {historyState.redoDescription && (
+                <p className="text-xs text-info-foreground/75 mt-1">
+                  Next redo: {historyState.redoDescription}
+                </p>
+              )}
+            </div>
+          </div>
+        </div>
+      )}
+
+      {/* Control Buttons */}
+      <div className="grid grid-cols-2 gap-2">
+        <Button
+          variant="outline"
+          onClick={onUndo}
+          disabled={!historyState.canUndo}
+          title={
+            historyState.canUndo
+              ? `Undo ${historyState.undoDescription} (Ctrl+Z)`
+              : 'Nothing to undo (Ctrl+Z)'
+          }
+          className="justify-start"
+        >
+          <Undo2 className="h-4 w-4 mr-2" />
+          Undo
+        </Button>
+
+        <Button
+          variant="outline"
+          onClick={onRedo}
+          disabled={!historyState.canRedo}
+          title={
+            historyState.canRedo
+              ? `Redo ${historyState.redoDescription} (Ctrl+Y)`
+              : 'Nothing to redo (Ctrl+Y)'
+          }
+          className="justify-start"
+        >
+          <Redo2 className="h-4 w-4 mr-2" />
+          Redo
+        </Button>
+
+        {onClear && historyState.historySize > 0 && (
+          <Button
+            variant="outline"
+            onClick={onClear}
+            title="Clear history"
+            className="justify-start col-span-2"
+          >
+            <History className="h-4 w-4 mr-2" />
+            Clear History ({historyState.historySize})
+          </Button>
+        )}
+      </div>
+
+      {/* Keyboard Shortcuts Info */}
+      <div className="text-xs text-muted-foreground space-y-1 p-3 rounded-lg bg-muted/30">
+        <p className="font-medium mb-2">History Shortcuts:</p>
+        <p>• Ctrl+Z: Undo last action</p>
+        <p>• Ctrl+Y or Ctrl+Shift+Z: Redo last action</p>
+        <p className="mt-2 text-xs text-muted-foreground/75">
+          History tracks up to 50 edit operations
+        </p>
+      </div>
+    </div>
+  );
+}