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>

.gitignore

@@ -32,6 +32,9 @@ embeddings/
 checkpoints/
 imported-decks/
 
+# User-created decks (generated at runtime, not committed)
+user-decks/*.dck
+
 # OS
 .DS_Store
 Thumbs.db