Top-level imports; catch only expected exceptions

velopack/tracerite/fastapi-vue are always available in the GUI build, so
drop the in-function import guards. The update checker now catches only
RuntimeError (not a Velopack install: dev/portable) and OSError
(network). Build script and PyInstaller spec use platformdirs for the
persistent tool cache.

scripts/guibuild.py

@@ -31,6 +31,7 @@ import zipfile
 from pathlib import Path
 
 import setuptools_scm
+from platformdirs import user_cache_path
 
 # BtbN automated builds always publish a 'latest' tag with this asset.
 _FFMPEG_URL = (
@@ -46,11 +47,7 @@ _ASSETS_DIR = _REPO_ROOT / "mediahive" / "assets"
 
 def _build_cache_dir() -> Path:
     """Return the persistent cross-build cache dir for downloaded tools (CI wipes build/)."""
-    if sys.platform == "win32":
-        base = os.environ.get("LOCALAPPDATA") or (Path.home() / "AppData" / "Local")
-        return Path(base) / "mediahive-build"
-    base = os.environ.get("XDG_CACHE_HOME") or (Path.home() / ".cache")
-    return Path(base) / "mediahive-build"
+    return user_cache_path("mediahive-build")
 
 
 _FFMPEG_STAGING = _build_cache_dir() / "ffmpeg"