Add clearer fallback messaging for YouTube anti-bot failures

app.py

@@ -84,7 +84,19 @@ def download_url(url):
             capture_output=True, text=True, timeout=600,
         )
         if result.returncode != 0:
-            return None, f"Download failed: {result.stderr.strip()}"
+            err = (result.stderr or '').strip()
+            lowered = err.lower()
+            if 'sign in to confirm you\'re not a bot' in lowered or 'cookies-from-browser' in lowered or 'cookies' in lowered:
+                return None, (
+                    'YouTube blocked anonymous extraction for this video. '
+                    'Please fall back to direct upload, or use a Google Drive / Dropbox / direct file link instead.'
+                )
+            if 'no supported javascript runtime could be found' in lowered:
+                return None, (
+                    'This source needs a JavaScript runtime for extraction. '
+                    'Please try again shortly, or fall back to direct upload / non-YouTube hosted links.'
+                )
+            return None, f"Download failed: {err}"
         import glob
         files = glob.glob(tmp_path.replace(".%(ext)s", "*"))
         if files: