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/utils/platform_helper.py

@@ -261,6 +261,37 @@ class PlatformHelper:
         else:
             return PlatformHelper.get_config_dir() / "cache"
 
+    @staticmethod
+    def get_data_dir() -> Path:
+        r"""
+        Get appropriate data directory for current platform
+
+        Used for application state that should persist across runs and is not
+        a transient cache (e.g. the JLCPCB parts database).
+
+        Follows platform conventions:
+        - Windows: %USERPROFILE%\.kicad-mcp\data
+        - Linux: $XDG_DATA_HOME/kicad-mcp or ~/.local/share/kicad-mcp
+        - macOS: ~/Library/Application Support/kicad-mcp
+
+        Returns:
+            Path to data directory
+        """
+        if PlatformHelper.is_windows():
+            return PlatformHelper.get_config_dir() / "data"
+        elif PlatformHelper.is_linux():
+            xdg_data = os.environ.get("XDG_DATA_HOME")
+            if xdg_data:
+                xdg_data_path = Path(xdg_data).expanduser()
+                if xdg_data_path.is_absolute():
+                    return xdg_data_path / "kicad-mcp"
+                logger.warning("Ignoring relative XDG_DATA_HOME: %s", xdg_data)
+            return Path.home() / ".local" / "share" / "kicad-mcp"
+        elif PlatformHelper.is_macos():
+            return Path.home() / "Library" / "Application Support" / "kicad-mcp"
+        else:
+            return PlatformHelper.get_config_dir() / "data"
+
     @staticmethod
     def ensure_directories() -> None:
         """Create all necessary directories if they don't exist"""
@@ -268,6 +299,7 @@ class PlatformHelper:
             PlatformHelper.get_config_dir(),
             PlatformHelper.get_log_dir(),
             PlatformHelper.get_cache_dir(),
+            PlatformHelper.get_data_dir(),
         ]
 
         for directory in dirs_to_create:
@@ -317,6 +349,7 @@ def detect_platform() -> dict:
         "config_dir": str(PlatformHelper.get_config_dir()),
         "log_dir": str(PlatformHelper.get_log_dir()),
         "cache_dir": str(PlatformHelper.get_cache_dir()),
+        "data_dir": str(PlatformHelper.get_data_dir()),
         "kicad_python_paths": [str(p) for p in PlatformHelper.get_kicad_python_paths()],
     }