feat: 字幕轨名称可读化

外挂字幕原来直接拿文件名的语言后缀当名称（chs、chs[BilibiliID_danmu]），
内嵌字幕没有 title 标签时也是裸语言码（chi）。改成统一映射成中文名：
chi/zh -> 中文，chs -> 简体中文，jpn -> 日本語，并把弹幕转存的
[BilibiliID_danmu] 文件标注为「弹幕」，避免和对白字幕混淆。

认不出来的标记保持原样（含 und），Emby 客户端的「字幕 (ASS)」兜底不变。

internal/service/subtitle.go

@@ -182,6 +182,77 @@ func (s *SubtitleService) discoverExternalUncached(ctx context.Context, mediaID
 	return discoverExternalSubtitleTracks(m), nil
 }
 
+// subtitleLanguageNames 把文件名后缀/容器里的语言标记折成可读名称。
+//
+// 外挂字幕只能从文件名拿到语言（`movie.chs.ass` → `chs`），直接展示给用户
+// 就是「chs」这种看不懂的值；内嵌轨没有 title 标签时同理。认不出来的标记
+// 原样保留，交给调用方（例如 Emby 的「字幕 (ASS)」兜底）处理。
+var subtitleLanguageNames = map[string]string{
+	"zh":  "中文",
+	"chi": "中文",
+	"zho": "中文",
+	"cn":  "中文",
+
+	"chs":     "简体中文",
+	"sc":      "简体中文",
+	"zh-cn":   "简体中文",
+	"zh-hans": "简体中文",
+	"zh-sg":   "简体中文",
+
+	"cht":     "繁体中文",
+	"tc":      "繁体中文",
+	"zh-tw":   "繁体中文",
+	"zh-hk":   "繁体中文",
+	"zh-mo":   "繁体中文",
+	"zh-hant": "繁体中文",
+
+	"jpn": "日本語",
+	"ja":  "日本語",
+	"jp":  "日本語",
+
+	"eng": "English",
+	"en":  "English",
+
+	"kor": "한국어",
+	"ko":  "한국어",
+
+	"yue": "粤语",
+}
+
+// subtitleLanguageName 返回语言标记的可读名称，认不出来时返回空串。
+func subtitleLanguageName(raw string) string {
+	tag := strings.ToLower(strings.TrimSpace(strings.ReplaceAll(raw, "_", "-")))
+	if tag == "" {
+		return ""
+	}
+	if name, ok := subtitleLanguageNames[tag]; ok {
+		return name
+	}
+	// `chs[bilibiliid_danmu]`、`chs.ass` 这类带附加信息的标记先取前缀。
+	head := tag
+	if i := strings.IndexAny(head, "[.@"); i > 0 {
+		head = strings.TrimSpace(head[:i])
+	}
+	return subtitleLanguageNames[head]
+}
+
+// subtitleDisplayLabel 把语言标记换成可读名称；认不出来时保留原值（含 und）。
+func subtitleDisplayLabel(raw string) string {
+	if name := subtitleLanguageName(raw); name != "" {
+		return name
+	}
+	return strings.TrimSpace(raw)
+}
+
+// isDanmakuSubtitle 报告字幕文件是否是弹幕转存（`xxx[BilibiliID_danmu].ass`）。
+// 这类轨道是飘过的弹幕而不是对白字幕，名称里要标出来，避免和对白字幕混淆。
+func isDanmakuSubtitle(name string) bool {
+	lower := strings.ToLower(name)
+	return strings.Contains(lower, "danmu") ||
+		strings.Contains(lower, "danmaku") ||
+		strings.Contains(name, "弹幕")
+}
+
 func discoverExternalSubtitleTracks(m *model.Media) []SubtitleTrack {
 	dir := filepath.Dir(m.Path)
 	bases := mediaSidecarBaseVariants(m.Path)
@@ -228,9 +299,13 @@ func discoverExternalSubtitleTracks(m *model.Media) []SubtitleTrack {
 				matchedBase = bases[0]
 			}
 			lang := detectLang(fullName, matchedBase)
+			label := subtitleDisplayLabel(lang)
+			if isDanmakuSubtitle(e.Name()) {
+				label += " · 弹幕"
+			}
 			tracks = append(tracks, SubtitleTrack{
 				Lang:     lang,
-				Label:    lang,
+				Label:    label,
 				Path:     filepath.Join(c, e.Name()),
 				Codec:    codec,
 				Source:   "external",
@@ -319,7 +394,11 @@ func subtitleTracksFromProbe(probe embeddedSubtitleProbe) []SubtitleTrack {
 		}
 		label := strings.TrimSpace(stream.Tags.Title)
 		if label == "" {
-			label = lang
+			// 没有 title 标签时用可读语言名（chi → 中文）代替裸语言码。
+			label = subtitleDisplayLabel(lang)
+		}
+		if label == "" {
+			label = "字幕"
 		}
 		if stream.Disposition.Forced != 0 {
 			label += "（强制）"

internal/service/subtitle_test.go

@@ -202,3 +202,103 @@ func TestSubtitleServeRawWritesSourceBytes(t *testing.T) {
 		t.Fatal("ServeRaw accepted a non-subtitle file")
 	}
 }
+
+func TestSubtitleLanguageNameMapsCommonTags(t *testing.T) {
+	cases := map[string]string{
+		"chs":                   "简体中文",
+		"CHS":                   "简体中文",
+		"zh-Hans":               "简体中文",
+		"cht":                   "繁体中文",
+		"chi":                   "中文",
+		"zh":                    "中文",
+		"jpn":                   "日本語",
+		"eng":                   "English",
+		"kor":                   "한국어",
+		"chs[bilibiliid_danmu]": "简体中文",
+		"und":                   "",
+		"":                      "",
+	}
+	for raw, want := range cases {
+		if got := subtitleLanguageName(raw); got != want {
+			t.Errorf("subtitleLanguageName(%q) = %q, want %q", raw, got, want)
+		}
+	}
+}
+
+func TestSubtitleDisplayLabelKeepsUnknownTags(t *testing.T) {
+	// 认不出来的标记原样返回：Emby 的 DisplayTitle 依赖 `und` 走「字幕 (ASS)」兜底。
+	if got := subtitleDisplayLabel("und"); got != "und" {
+		t.Fatalf("subtitleDisplayLabel(und) = %q, want und", got)
+	}
+	if got := subtitleDisplayLabel("weird"); got != "weird" {
+		t.Fatalf("subtitleDisplayLabel(weird) = %q, want weird", got)
+	}
+}
+
+func TestIsDanmakuSubtitle(t *testing.T) {
+	if !isDanmakuSubtitle("S01E01.chs[BilibiliID_danmu].ass") {
+		t.Fatal("danmaku sidecar was not detected")
+	}
+	if !isDanmakuSubtitle("弹幕.ass") {
+		t.Fatal("Chinese danmaku name was not detected")
+	}
+	if isDanmakuSubtitle("S01E01.chs.ass") {
+		t.Fatal("normal subtitle was misdetected as danmaku")
+	}
+}
+
+func TestDiscoverExternalSubtitleLabelsAreReadable(t *testing.T) {
+	dir := t.TempDir()
+	videoPath := filepath.Join(dir, "S01E01.mkv")
+	files := map[string]string{
+		"S01E01.chs.ass":                   "[Script Info]\n",
+		"S01E01.chs[BilibiliID_danmu].ass": "[Script Info]\n",
+	}
+	for name, body := range files {
+		if err := os.WriteFile(filepath.Join(dir, name), []byte(body), 0o600); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	m := &model.Media{Path: videoPath}
+	tracks := discoverExternalSubtitleTracks(m)
+	if len(tracks) != 2 {
+		t.Fatalf("len(tracks) = %d, want 2: %#v", len(tracks), tracks)
+	}
+	labels := map[string]string{}
+	for _, track := range tracks {
+		labels[filepath.Base(track.Path)] = track.Label
+	}
+	if got := labels["S01E01.chs.ass"]; got != "简体中文" {
+		t.Fatalf("chs.ass label = %q, want 简体中文", got)
+	}
+	if got := labels["S01E01.chs[BilibiliID_danmu].ass"]; got != "简体中文 · 弹幕" {
+		t.Fatalf("danmaku label = %q, want 简体中文 · 弹幕", got)
+	}
+}
+
+func TestEmbeddedSubtitleProbeUsesReadableLanguage(t *testing.T) {
+	var probe embeddedSubtitleProbe
+	raw := []byte(`{"streams":[
+		{"index":2,"codec_name":"hdmv_pgs_subtitle","tags":{"language":"chi"},"disposition":{"default":1}},
+		{"index":3,"codec_name":"subrip","tags":{"language":"jpn"}},
+		{"index":4,"codec_name":"subrip","tags":{"language":"chi","title":"简日双语"}}
+	]}`)
+	if err := json.Unmarshal(raw, &probe); err != nil {
+		t.Fatal(err)
+	}
+	tracks := subtitleTracksFromProbe(probe)
+	if len(tracks) != 3 {
+		t.Fatalf("len(tracks) = %d, want 3", len(tracks))
+	}
+	if got := tracks[0].Label; got != "中文（默认）（内嵌·图片）" {
+		t.Fatalf("bitmap label = %q", got)
+	}
+	if got := tracks[1].Label; got != "日本語（内嵌）" {
+		t.Fatalf("text label = %q", got)
+	}
+	// 容器自带 title 时优先用 title。
+	if got := tracks[2].Label; got != "简日双语（内嵌）" {
+		t.Fatalf("titled label = %q", got)
+	}
+}