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/domain/types.go

@@ -118,7 +118,6 @@ type DownloadJob struct {
 	MediaType      MediaType
 	SelectedFormat *Format
 	FormatString   string
-	Container      string
 	Language       string
 	Status         DownloadStatus
 	Progress       float64