feat: separate writable data dir from program dir (LAN update groundwork)

- add get_data_dir() with CMBOT_DATA_DIR override, fallback to program dir
- point config/logs/output at the data root; resources stay on program root
- harden config/template writes with parents=True for a deep data root
- add docs/10-lan-update.md (LAN auto-update design); default install root
  %LOCALAPPDATA%\CMBot for elevation-free self-update

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/services/template_service.py

@@ -77,7 +77,7 @@ def _save_custom_templates(templates: List[Template]):
     """Write custom templates to JSON. Logs error on failure, does not raise."""
     path = _templates_file()
     try:
-        path.parent.mkdir(exist_ok=True)
+        path.parent.mkdir(parents=True, exist_ok=True)
         payload = {
             "templates": [
                 {