Fix subtitle positioning and show existing langs without re-downloading

- Strip yt-dlp's align:start position:0% cue settings from VTT files
  after both video download and subtitle-only download so CSS ::cue centers them
- CC chip now shows already-downloaded langs (e.g. 'CC: en') directly
  from disk with a '+' button to add more — no YouTube call needed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/ytdlp.py

@@ -384,6 +384,25 @@ def fetch_channel_links(channel_id: str) -> list[str]:
     return list(channel_ids)
 
 
+def _strip_vtt_cue_settings(video_id: str) -> None:
+    """Remove position/align/line cue settings from yt-dlp VTT files.
+
+    yt-dlp embeds 'align:start position:0%' in every cue header which pins
+    subtitles to the bottom-left. Stripping them lets CSS ::cue center them.
+    """
+    for vtt in Path(settings.download_path).glob(f"{video_id}.*.vtt"):
+        try:
+            text = vtt.read_text(encoding="utf-8", errors="replace")
+            cleaned = re.sub(
+                r'(\d{1,2}:\d{2}:\d{2}\.\d{3} --> \d{1,2}:\d{2}:\d{2}\.\d{3})[^\n]*',
+                r'\1',
+                text,
+            )
+            vtt.write_text(cleaned, encoding="utf-8")
+        except Exception:
+            pass
+
+
 def download_subs_only(video_id: str, subtitle_langs: str) -> bool:
     """Download subtitle files only (no video) for an already-downloaded video."""
     url = f"https://www.youtube.com/watch?v={video_id}"
@@ -397,6 +416,8 @@ def download_subs_only(video_id: str, subtitle_langs: str) -> bool:
         "-o", output_template,
         *_cookie_args(),
     ], timeout=60)
+    if code == 0:
+        _strip_vtt_cue_settings(video_id)
     return code == 0
 
 
@@ -721,6 +742,7 @@ def start_download(
 
             process.wait()
             if process.returncode == 0:
+                _strip_vtt_cue_settings(video_id)
                 resolution = detect_resolution(file_path) if file_path else None
                 on_complete(download_id, file_path, resolution)
             else: