feat: deck dropdown, user-decks pipeline, deckgen fixes, gpt-oss removal

- Add user-decks/ directory and USER_DECKS_DIR config for exported decks
- Update export-to-sim to write to user-decks/ with static mount in lab_api
- Replace import/paste UI with API-backed "My Decks" dropdown in LAN client
  and AI Lab, fetching from GET /api/lab/decks with source grouping
- Fix GET /api/lab/decks to JOIN deck_cards for totalCards count
- Fix deckgenerator: add missing DOM elements (dg-strategy-bar, dg-stats-bar,
  dg-card-groups) inside #dg-results, null guards on render functions,
  fix setLoading always hiding results
- Replace gpt-oss:20b with claude-opus-4-20250514 across all files
- Add user-decks/ to _BUILTIN_DECK_DIRS in deck_service.py
- Enrich missing type_line from Forge card cache in DB deck path
- Collapse import bar behind <details> toggle in AI Lab

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lab_api.py

@@ -170,6 +170,7 @@ from pathlib import Path as _Path
 _legacy_ui_dir = _Path(__file__).parent / "ui"
 _lan_client_dir = _Path(__file__).parent / "lan-client"
 _precon_decks_dir = _Path(__file__).parent / "precon-decks"
+_user_decks_dir = _Path(__file__).parent / "user-decks"
 _spa_dir = _Path(__file__).parent / "frontend" / "commander-ai-lab-ui" / "dist"
 
 # Mount LAN client at /lan (served before the root catch-all)
@@ -180,6 +181,10 @@ if _lan_client_dir.exists():
 if _precon_decks_dir.exists():
     app.mount("/precon-decks", StaticFiles(directory=str(_precon_decks_dir)), name="precon-decks")
 
+# Mount user-decks/ so exported .dck files are fetchable at /user-decks/{file}.dck
+if _user_decks_dir.exists():
+    app.mount("/user-decks", StaticFiles(directory=str(_user_decks_dir)), name="user-decks")
+
 if _legacy_ui_dir.exists():
     app.mount("/", StaticFiles(directory=str(_legacy_ui_dir), html=True), name="ui")
 elif _spa_dir.exists():