feat: implement medium effort features - markers, web workers, and bezier automation

Implemented three major medium effort features to enhance the audio editor:

**1. Region Markers System**
- Add marker type definitions supporting point markers and regions
- Create useMarkers hook for marker state management
- Build MarkerTimeline component for visual marker display
- Create MarkerDialog component for adding/editing markers
- Add keyboard shortcuts: M (add marker), Shift+M (next), Shift+Ctrl+M (previous)
- Support marker navigation, editing, and deletion

**2. Web Worker for Computations**
- Create audio worker for offloading heavy computations
- Implement worker functions: generatePeaks, generateMinMaxPeaks, normalizePeaks, analyzeAudio, findPeak
- Build useAudioWorker hook for easy worker integration
- Integrate worker into Waveform component with peak caching
- Significantly improve UI responsiveness during waveform generation

**3. Bezier Curve Automation**
- Enhance interpolateAutomationValue to support Bezier curves
- Implement cubic Bezier interpolation with control handles
- Add createSmoothHandles for auto-smooth curve generation
- Add generateBezierCurvePoints for smooth curve rendering
- Support bezier alongside existing linear and step curves

All features are type-safe and integrate seamlessly with the existing codebase.

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

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

lib/hooks/useMarkers.ts

@@ -0,0 +1,70 @@
+'use client';
+
+import { useState, useCallback } from 'react';
+import type { Marker, CreateMarkerInput } from '@/types/marker';
+
+export function useMarkers() {
+  const [markers, setMarkers] = useState<Marker[]>([]);
+
+  const addMarker = useCallback((input: CreateMarkerInput): Marker => {
+    const marker: Marker = {
+      ...input,
+      id: `marker-${Date.now()}-${Math.random()}`,
+    };
+    setMarkers((prev) => [...prev, marker].sort((a, b) => a.time - b.time));
+    return marker;
+  }, []);
+
+  const updateMarker = useCallback((id: string, updates: Partial<Marker>) => {
+    setMarkers((prev) => {
+      const updated = prev.map((m) =>
+        m.id === id ? { ...m, ...updates } : m
+      );
+      // Re-sort if time changed
+      if ('time' in updates) {
+        return updated.sort((a, b) => a.time - b.time);
+      }
+      return updated;
+    });
+  }, []);
+
+  const removeMarker = useCallback((id: string) => {
+    setMarkers((prev) => prev.filter((m) => m.id !== id));
+  }, []);
+
+  const clearMarkers = useCallback(() => {
+    setMarkers([]);
+  }, []);
+
+  const getMarkerAt = useCallback((time: number, tolerance: number = 0.1): Marker | undefined => {
+    return markers.find((m) => {
+      if (m.type === 'point') {
+        return Math.abs(m.time - time) <= tolerance;
+      } else {
+        // For regions, check if time is within the region
+        return m.endTime !== undefined && time >= m.time && time <= m.endTime;
+      }
+    });
+  }, [markers]);
+
+  const getNextMarker = useCallback((time: number): Marker | undefined => {
+    return markers.find((m) => m.time > time);
+  }, [markers]);
+
+  const getPreviousMarker = useCallback((time: number): Marker | undefined => {
+    const previous = markers.filter((m) => m.time < time);
+    return previous[previous.length - 1];
+  }, [markers]);
+
+  return {
+    markers,
+    addMarker,
+    updateMarker,
+    removeMarker,
+    clearMarkers,
+    getMarkerAt,
+    getNextMarker,
+    getPreviousMarker,
+    setMarkers,
+  };
+}