fix: add _resolve_precon_dir() auto-detection so CFG.precon_dir is not empty

The precon deck list was missing after the monolith refactor because
PRECON_DIR was hardcoded in services/precon_service.py with no CLI arg,
env var, or auto-resolve fallback — unlike forge_jar, forge_dir, etc.

Changes:
- Add precon_dir field to Config class (models/state.py)
- Add --precon-dir CLI arg and PRECON_DIR env var support
- Add _resolve_precon_dir() with project-root, parent-dir, and
  forge_dir-relative detection; creates dir as fallback for downloads
- Refactor precon_service.py to use _get_precon_dir() helper that reads
  CFG.precon_dir at runtime instead of a hardcoded module-level Path
- Update routes/precon.py to use _get_precon_dir() for install endpoints
- Wire _on_startup to resolve precon_dir for uvicorn-direct launches
- Remove unused PRECON_DIR/PRECON_INDEX imports from routes/lab.py
- Audit confirms all other CFG path fields already have fallbacks

routes/lab.py

@@ -38,8 +38,6 @@ from routes.shared import (
     StartResponse,
     StatusResponse,
     AI_PROFILES,
-    PRECON_DIR,
-    PRECON_INDEX,
     build_java_command,
     run_batch_subprocess,
     _run_deepseek_batch_thread,

routes/precon.py

@@ -16,7 +16,8 @@ from pathlib import Path
 from fastapi import APIRouter, HTTPException
 
 import routes.shared as _shared                    # module reference, not value import
-from routes.shared import CFG, PRECON_DIR, download_precon_database, log
+from routes.shared import CFG, download_precon_database, log
+from services.precon_service import _get_precon_dir
 
 router = APIRouter(tags=["precon"])
 
@@ -36,7 +37,7 @@ async def install_precon(req: dict):
     if not file_name:
         raise HTTPException(400, "fileName is required")
 
-    src = PRECON_DIR / file_name
+    src = _get_precon_dir() / file_name
     if not src.exists():
         raise HTTPException(404, f"Precon not found: {file_name}")
 
@@ -71,7 +72,7 @@ async def install_precons_batch(req: dict):
 
     results = []
     for file_name in file_names:
-        src = PRECON_DIR / file_name
+        src = _get_precon_dir() / file_name
         if not src.exists():
             results.append({"fileName": file_name, "installed": False, "error": "not found"})
             continue

routes/shared.py

@@ -186,6 +186,7 @@ from services.import_service import (
 from services.precon_service import (
     PRECON_DIR, PRECON_INDEX, GITHUB_PRECON_URL, PRECON_CACHE_HOURS,
     load_precon_index, _sanitize_filename, _deck_to_dck, download_precon_database,
+    _get_precon_dir,
 )
 
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