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/gray_theme.css

@@ -9,6 +9,7 @@
     --thumb-background:      #35404D;
     --link:                  #22AAFF;
     --link-visited:          #7755FF;
+    --border-color:          #4A5568;
     --border-bg:             #FFFFFF;
     --border-bg-settings:    #FFFFFF;
     --border-bg-license:     #FFFFFF;