refactor: remove container selection, extract helpers, add logging and tests

- Remove container picker UI, keyboard, settings, and ContainerOptions
  entirely — yt-dlp handles container format automatically
- Remove Container field from DownloadJob struct and related references
- Extract buildFormatList helper to deduplicate format list building
  between handleURLInput and handleBackStep
- Break runDownload into trackDownloadProgress + handleDownloadCompletion
- Add HTTP timeout (30s) to downloadFile helper
- Log applyQuota and AddHistory errors instead of discarding
- Log nil stepState warnings in all handler entry points
- Include job.StderrLog in download failure messages
- Add tests for buildFormatList, findFormatByID, determineMediaType,
  and buildFormatString
- Update README with search, DRM fallback, and quota feature docs
- Remove unused strings import from keyboard.go

internal/dl/ytdlp.go

@@ -265,28 +265,17 @@ func (c *Client) StartDownload(job *domain.DownloadJob, downloadDir string) (<-c
 	formatID := job.SelectedFormat.ID
 	var args []string
 
-	// Use explicit format string if set (video+audio combination)
+	// Use format string for the download — yt-dlp handles container choice
 	if job.FormatString != "" {
 		args = append(args, "-f", job.FormatString)
-		if job.Container != "" {
-			args = append(args, "--merge-output-format", job.Container)
-		}
 	} else {
-		// Build format string for merge
 		switch job.MediaType {
 		case domain.MediaTypeAudio:
 			args = append(args, "-f", formatID, "-x")
-			if job.Container != "" {
-				args = append(args, "--audio-format", job.Container)
-			}
 		case domain.MediaTypeVideo:
 			args = append(args, "-f", formatID)
 		case domain.MediaTypeVideoAudio:
-			// Download best video + best audio that match
 			args = append(args, "-f", formatID+"+bestaudio/best")
-			if job.Container != "" {
-				args = append(args, "--merge-output-format", job.Container)
-			}
 		}
 	}
 
@@ -296,7 +285,7 @@ func (c *Client) StartDownload(job *domain.DownloadJob, downloadDir string) (<-c
 
 	// Progress reporting using progress-template for machine-readable output
 	args = append(args, "--newline")
-	args = append(args, "--progress-template", "stderr:DLPROG|%(progress.percent)s|%(progress._speed_str)s|%(progress._eta_str)s")
+	args = append(args, "--progress-template", "stderr:DLPROG|%(progress.percent)s|%(progress.speed)s|%(progress.eta)s")
 
 	// Language selection if specified
 	if job.Language != "" {
@@ -527,17 +516,3 @@ func formatLabel(f domain.Format) string {
 	}
 	return f.ID
 }
-
-// ContainerOptions returns the container format options for a given media type.
-func ContainerOptions(mt domain.MediaType) []string {
-	switch mt {
-	case domain.MediaTypeAudio:
-		return []string{"mp3", "m4a", "opus"}
-	case domain.MediaTypeVideo:
-		return []string{"mp4", "mkv", "webm"}
-	case domain.MediaTypeVideoAudio:
-		return []string{"mp4", "mkv"}
-	default:
-		return []string{"mp4"}
-	}
-}