feat(config): unify mpv plugin options under main config and add CSS/Ani

- Replace subminer.conf plugin config with mpv.* fields in config.jsonc
- Add socketPath, backend, autoStartSubMiner, pauseUntilOverlayReady, aniskipEnabled/buttonKey, subminerBinaryPath to mpv config
- Add subtitleSidebar.css field; migrate legacy sidebar appearance fields
- Add paintOrder and WebkitTextStroke to subtitle style options
- Update default subtitle/sidebar fontFamily to CJK-first stack
- Fix overlay visible state surviving mpv y-r restart
- Fix live config saves applying subtitle CSS immediately to open overlays
- Migrate legacy primary/secondary subtitle appearance into subtitleStyle.css on load
- Switch AniSkip button key setting to click-to-learn key capture

launcher/config/args-normalizer.ts

@@ -118,7 +118,7 @@ export function createDefaultArgs(
   const youtubeAudioLangs = uniqueNormalizedLangCodes([...primarySubLangs, ...secondarySubLangs]);
 
   const parsed: Args = {
-    backend: 'auto',
+    backend: mpvConfig.backend ?? 'auto',
     directory: '.',
     recursive: false,
     profile: '',