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/repo/store_test.go

@@ -59,8 +59,8 @@ func TestPreferences(t *testing.T) {
 	if err != nil {
 		t.Fatalf("GetOrCreatePreferences() failed: %v", err)
 	}
-	if prefs.DefaultMediaType != domain.MediaTypeVideoAudio {
-		t.Errorf("default media type = %v, want VideoAudio", prefs.DefaultMediaType)
+	if prefs.DefaultMediaType != domain.MediaTypeAsk {
+		t.Errorf("default media type = %v, want Ask", prefs.DefaultMediaType)
 	}
 
 	prefs.DefaultMediaType = domain.MediaTypeAudio