Fix update checker: find git despite systemd minimal PATH

shutil.which("git") fails under systemd's stripped PATH. Added
_find_git() that checks common locations (/usr/bin/git, etc.)
as fallback. Also added null-safe check on apiFetch response
in the update checker frontend code.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/routers/updates.py

@@ -16,7 +16,20 @@ router = APIRouter(prefix="/api/updates", tags=["updates"])
 # ---------------------------------------------------------------------------
 
 _IS_GIT = (BASE_DIR / ".git").is_dir()
-_HAS_GIT = bool(shutil.which("git"))
+
+
+def _find_git() -> bool:
+    """Check if git is available, even with systemd's minimal PATH."""
+    if shutil.which("git"):
+        return True
+    # systemd services often have a stripped PATH — check common locations
+    for p in ("/usr/bin/git", "/usr/local/bin/git", "/bin/git"):
+        if os.path.isfile(p) and os.access(p, os.X_OK):
+            return True
+    return False
+
+
+_HAS_GIT = _find_git()
 
 
 def _require_git():

frontend/js/atlus.js

@@ -397,7 +397,7 @@
         if (updateDismissed) return;
         try {
             const res = await Atlus.apiFetch('/api/updates/check');
-            if (!res.ok) return;
+            if (!res || !res.ok) return;
             const data = await res.json();
             const panel = $('#panelUpdates');
             if (!panel) return;