Root cause: dashboard.router registered BEFORE board.router in main.py,
so dashboard's /pages/{page_id} handler got all requests and ignored
the embed parameter. board.py's embed-aware route was never reached.
Fix: add embed support to dashboard.py's view_page_root too:
- Detect ?embed=1 query param
- Select page_editor_embed.html when embed=True
- Pass embed_mode to template context