feat: implement ffmpeg.rs, subtitles.rs, and fonts.rs

- ffmpeg.rs: discovery, duration extraction, metadata probing, download
- subtitles.rs: SRT-to-VTT conversion, sidecar discovery, storage, extraction
- fonts.rs: Google Fonts and Font Awesome local caching

src-tauri/src/lib.rs

@@ -1,6 +1,9 @@
+pub mod ffmpeg;
+pub mod fonts;
 pub mod prefs;
 pub mod recents;
 pub mod state;
+pub mod subtitles;
 pub mod utils;
 
 #[cfg_attr(mobile, tauri::mobile_entry_point)]