Switch to android_vr player client — no Node.js required

android_vr provides pre-signed format URLs that bypass YouTube's
n-challenge and signature JS requirements entirely. Tested: 23 video
formats available without any JavaScript runtime installed.

Reverts Node.js Dockerfile addition (which failed to build anyway).

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

backend/routers/settings.py

@@ -247,7 +247,7 @@ def ytdlp_test(
             "yt-dlp",
             "https://www.youtube.com/watch?v=dQw4w9WgXcQ",
             "--dump-json", "--no-download", "--no-playlist",
-            "--extractor-args", "youtube:player_client=web",
+            "--extractor-args", "youtube:player_client=android_vr",
             *cookie_args,
         ],
         capture_output=True, text=True, timeout=30,