feat: style native Cast subtitles

README.md

@@ -63,6 +63,8 @@ Open media, select the **Cast** icon or **Playback → Cast to Device → Find C
 
 Existing controls operate remote playback. **Play on This Mac** returns to local playback at the receiver's position. Opening different media ends casting. Frame stepping and YouTube quality changes are local-only; live Cast playback is not seekable. Cast output is limited to 1080p/30 fps. The screen-and-triangle icon does not provide AirPlay support.
 
+External Cast subtitles use white sans-serif text with a black outline, transparent background and no enclosing window. Font scale currently uses the receiver default (1.0), pending physical-TV tuning. Language selection and Off remain independent of video delivery; no subtitle burn-in is used.
+
 Cast trusts the selected LAN endpoint's self-signed certificate; use it only on a trusted network. Receiver captions and perceived A/V synchronization still need physical-device verification.
 
 ## Command line

Sources/App/CastSession.swift

@@ -421,6 +421,16 @@ final class CastSession {
         ]
         if media.duration > 0, !media.live { info["duration"] = media.duration }
         if !media.tracks.isEmpty {
+            info["textTrackStyle"] = [
+                "foregroundColor": "#FFFFFFFF",
+                "backgroundColor": "#00000000",
+                "edgeType": "OUTLINE",
+                "edgeColor": "#000000FF",
+                "fontGenericFamily": "SANS_SERIF",
+                "fontStyle": "NORMAL",
+                "fontScale": 1.0, // Receiver default; tune against physical TV output.
+                "windowType": "NONE"
+            ]
             info["tracks"] = media.tracks.map { track -> [String: Any] in
                 ["trackId": track.id, "type": "TEXT", "subtype": "SUBTITLES",
                  "trackContentId": track.url.absoluteString, "trackContentType": "text/vtt",

TODO.md

@@ -1,5 +1,5 @@
-- [ ] improve chrome cast subtitle rendering
-    - maybe this will require stamping the subtitles into the video frames manually before submission to the chrome cast (depending on what it supports)
+- [x] improve chrome cast subtitle rendering
+    - native WebVTT style: white sans-serif, black outline, transparent background; physical-TV font-scale tuning remains unverified
 - [ ] add playlist support
     - maybe by allowing multiple file selection? 🤔
 - [ ] in the "choose what to play" screen, we should change the youtube button to general streaming