Backend:
- navi/profiles/loader.py: add save_profile_to_dir() to write config.json
  and system_prompt.txt back to disk
- navi/core/registry.py: add ProfileRegistry.update() for in-memory updates
- navi/api/routes/admin.py: new endpoints GET /admin/profiles/{id} and
  PUT /admin/profiles/{id} for reading and saving full profile config

Admin panel:
- Profiles table: add Edit button per row
- Drawer form with sections: Basic, Model & Generation, Thinking,
  Planning, Sub-agent, Tools, System Prompt
- All fields editable inline (text inputs, checkboxes, textareas for lists)
- Save via PUT request, updates both disk and in-memory registry without
  server restart

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>