refactor: convert download dropdown to valid W3C table

- Replace ul/li structure with semantic table (thead/tbody)
- Add 6 columns: Ext, Video, Audio, Size, Codecs, Link
- Show "N/A" for missing codec data
- Add 0.5rem spacing between dropdown and description

youtube/static/dark_theme.css

@@ -9,6 +9,7 @@
     --thumb-background:     #222222;
     --link:                 #00B0FF;
     --link-visited:         #40C4FF;
+    --border-color:         #333333;
     --border-bg:            #222222;
     --border-bg-settings:   #000000;
     --border-bg-license:    #000000;