Add video codec selector and codec/quality columns in done list

Allow users to prefer a specific video codec (H.264, H.265, AV1, VP9)
when adding downloads. The selector filters available formats via
yt-dlp format strings, falling back to best available if the preferred
codec is not found. The completed downloads table now shows Quality
and Codec columns.

ui/src/app/interfaces/download.ts

@@ -13,6 +13,7 @@ export interface Download {
   subtitle_format?: string;
   subtitle_language?: string;
   subtitle_mode?: string;
+  video_codec?: string;
   status: string;
   msg: string;
   percent: number;