docs: update PLAN.md with completed tasks

All main features implemented:
- CLI and Serve modes
- Markdown parsing with goldmark
- HTML templates with theme selection
- PDF generation with go-pdf/fpdf
- File watching with fsnotify
- Dockerfile for multi-stage build

Co-Authored-By: Claude (glm-5) <noreply@anthropic.com>

PLAN.md

@@ -15,13 +15,14 @@
 - [x] CLI Mode (`gocv`) generates static files to `./output` and exits.
 - [x] Serve Mode (`gocv serve`) hosts HTML and serves PDF on demand.
 - [x] File Watcher implemented for live reload in Serve Mode (using fsnotify).
-- [ ] Dockerfile created for multi-stage build.
+- [x] Dockerfile created for multi-stage build.
 
 ## Active Task
 - [x] Analyze existing backbone code and integrate Markdown parsing.
 - [x] Integrate HTML template engine with theme selection.
 - [x] Implement Serve Mode with file watching for live reload.
-- [ ] Create Dockerfile for multi-stage build.
+- [x] Create Dockerfile for multi-stage build.
+- [ ] All tasks complete - project ready for testing.
 
 ## Known Issues / Blockers
 - [x] Identify best Pure Go PDF library that supports HTML/CSS (or define CSS subset).