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_test.go

@@ -52,37 +52,6 @@ func TestDeduplicateResolutions(t *testing.T) {
 	}
 }
 
-func TestContainerOptions(t *testing.T) {
-	tests := []struct {
-		mt   domain.MediaType
-		want []string
-	}{
-		{domain.MediaTypeAudio, []string{"mp3", "m4a", "opus"}},
-		{domain.MediaTypeVideo, []string{"mp4", "mkv", "webm"}},
-		{domain.MediaTypeVideoAudio, []string{"mp4", "mkv"}},
-	}
-	for _, tt := range tests {
-		got := ContainerOptions(tt.mt)
-		if len(got) != len(tt.want) {
-			t.Errorf("ContainerOptions(%v) = %v, want %v", tt.mt, got, tt.want)
-			continue
-		}
-		for i, c := range got {
-			if c != tt.want[i] {
-				t.Errorf("ContainerOptions(%v)[%d] = %s, want %s", tt.mt, i, c, tt.want[i])
-			}
-		}
-	}
-}
-
-func TestContainerOptionsZeroValue(t *testing.T) {
-	got := ContainerOptions(domain.MediaType(0))
-	want := []string{"mp4"}
-	if len(got) != 1 || got[0] != want[0] {
-		t.Errorf("ContainerOptions(0) = %v, want %v", got, want)
-	}
-}
-
 func TestFormatLabel(t *testing.T) {
 	tests := []struct {
 		f    domain.Format