Validate announcement URL scheme and fix llama-only mode guard

- Only allow https:// and http:// URLs for announcement_url to prevent
  javascript: or data: scheme injection from a compromised manifest.

- Guard the install timestamp write with both _LLAMA_ONLY and
  _SKIP_PYTHON_DEPS checks to prevent uninitialized variable errors
  when running in UNSLOTH_STUDIO_LLAMA_ONLY=1 mode (set -u).

studio/backend/utils/update_check.py

@@ -94,7 +94,10 @@ def fetch_and_cache_update_status() -> UpdateStatus:
     if isinstance(announcement, dict):
         status.announcement_badge = announcement.get("badge") or None
         status.announcement_message = announcement.get("message") or None
-        status.announcement_url = announcement.get("url") or None
+        _url = announcement.get("url") or None
+        # Only allow http/https URLs to prevent javascript: or data: injection.
+        if _url and _url.startswith(("https://", "http://")):
+            status.announcement_url = _url
 
     _cached_status = status
     return status

studio/setup.sh

@@ -1062,7 +1062,8 @@ else
 fi  # end _SKIP_GGUF_BUILD check
 
 # ── Write install timestamp (only when python deps were actually updated) ──
-if [ "$_SKIP_PYTHON_DEPS" = false ]; then
+# Guard against uninitialized _SKIP_PYTHON_DEPS in llama-only mode.
+if [ "$_LLAMA_ONLY" != "1" ] && [ "${_SKIP_PYTHON_DEPS:-true}" = false ]; then
 _STUDIO_INFO_DIR="$HOME/.unsloth/studio"
 mkdir -p "$_STUDIO_INFO_DIR"
 python - "$_STUDIO_INFO_DIR/UNSLOTH_STUDIO_INFO.json" <<'PY' 2>/dev/null || true