chore: add PostCSS config and update implementation plan progress

- Added postcss.config.js for Tailwind CSS 4
- Updated PLAN.md with progress overview section
- Marked all completed features in implementation phases:
  - Phase 1: 95% complete (core infrastructure)
  - Phase 2: 90% complete (audio engine)
  - Phase 3: 60% complete (waveform visualization)
- Added deployment files to project structure documentation

Features marked as complete:
✅ Project setup with Next.js 16 + Tailwind CSS 4
✅ Core UI components (Button, Card, Slider, Progress, Toast)
✅ Theme system with dark/light mode
✅ Web Audio API integration
✅ Audio file upload and decoding
✅ Playback controls and volume management
✅ Waveform rendering with Canvas API
✅ Timeline scrubbing and seeking
✅ Real-time progress visualization

postcss.config.js

@@ -0,0 +1,8 @@
+/** @type {import('postcss-load-config').Config} */
+const config = {
+  plugins: {
+    '@tailwindcss/postcss': {},
+  },
+};
+
+module.exports = config;