feat: add auto-select from preferences and type picker

- Add MediaTypeAsk (0) as the default, mapping old video_audio to Ask
- Show Video/Audio/Ask type picker when no default media type is set
- Auto-select matching format and skip picker when defaults are configured
- Add autoSelectVideoFormat / autoSelectAudioFormat helpers with closest-match
- Add buildFilteredFormatList for type-filtered format lists
- Add Media Type setting in settings UI (Ask/Video/Audio)
- Add --merge-output-format mp4 for video downloads via yt-dlp
- Fix formatLabelForDisplay to show Height fallback and Extension
- Add cookies volume mount to docker-compose.yml
- Document cookies setup and new features in README

internal/dl/ytdlp.go

@@ -250,6 +250,11 @@ func (c *Client) StartDownload(job *domain.DownloadJob, downloadDir string) (<-c
 	outputPath := filepath.Join(downloadDir, job.ID+"_%(id)s.%(ext)s")
 	args = append(args, "--output", outputPath)
 
+	// Remux to mp4 for video output — Telegram-friendly container.
+	if job.MediaType != domain.MediaTypeAudio {
+		args = append(args, "--merge-output-format", "mp4")
+	}
+
 	// --progress forces progress output in non-TTY (pipe) environments
 	// --newline uses \n instead of \r so bufio.Scanner can read line-by-line
 	args = append(args, "--progress", "--newline")