fix(jlcpcb): use platform user data dir for parts database (#167)

JLCPCBPartsManager defaulted db_path to a "data/" directory computed
relative to __file__, which fails with read-only filesystems when the
package is installed to a system-managed prefix (e.g. /nix/store, an
immutable container image, or /usr/lib). The same pattern in
download_jlcpcb.py would silently scatter the ~1.5 GB JLCPCB cache
inside the install tree even when it is writable.

The original integration plan (docs/archive/JLCPCB_INTEGRATION_PLAN.md)
called for a per-user database under ~/.kicad-mcp/. This change moves
the default to the platform-appropriate user data directory by adding
a new PlatformHelper.get_data_dir() helper that mirrors the existing
get_config_dir() / get_cache_dir() conventions:

  - Linux:   XDG_DATA_HOME/kicad-mcp or ~/.local/share/kicad-mcp
  - macOS:   ~/Library/Application Support/kicad-mcp
  - Windows: %USERPROFILE%\.kicad-mcp\data

Both JLCPCBPartsManager and download_jlcpcb.py now resolve their
database paths through this helper. ensure_directories() and
detect_platform() include the new directory. Unit tests parallel to
the existing config_dir/cache_dir cases cover platform-appropriate
paths and the relative-XDG_DATA_HOME edge case.

python/commands/jlcpcb_parts.py

@@ -13,6 +13,8 @@ from datetime import datetime
 from pathlib import Path
 from typing import Any, Callable, Dict, List, Optional, Tuple
 
+from utils.platform_helper import PlatformHelper
+
 logger = logging.getLogger("kicad_interface")
 
 
@@ -28,13 +30,13 @@ class JLCPCBPartsManager:
         Initialize parts database manager
 
         Args:
-            db_path: Path to SQLite database file (default: data/jlcpcb_parts.db)
+            db_path: Path to SQLite database file (default: platform-specific
+                user data directory, e.g. ~/.local/share/kicad-mcp/jlcpcb_parts.db
+                on Linux). See PlatformHelper.get_data_dir() for platform paths.
         """
         if db_path is None:
-            # Default to data directory in project root
-            project_root = Path(__file__).parent.parent.parent
-            data_dir = project_root / "data"
-            data_dir.mkdir(exist_ok=True)
+            data_dir = PlatformHelper.get_data_dir()
+            data_dir.mkdir(parents=True, exist_ok=True)
             db_path = str(data_dir / "jlcpcb_parts.db")
 
         self.db_path = db_path