feat: implement video_protocol.rs, commands.rs, wire up main.rs, and index.html

- video_protocol.rs: tutdock:// custom protocol with HTTP Range support
  for video streaming, subtitle/font serving with path traversal protection
- commands.rs: all 26 Tauri command handlers as thin wrappers
- main.rs: full Tauri bootstrap with state management, window restore,
  async font caching, and ffmpeg discovery
- index.html: complete HTML markup extracted from Python app
- lib.rs: updated with all module declarations and AppPaths struct

src-tauri/src/library.rs

@@ -1438,7 +1438,7 @@ impl Library {
     /// This method signals any previous scan to stop, then prepares the
     /// pending list. The actual scanning is driven externally via
     /// `get_pending_scans()` and `apply_scanned_duration()`.
-    pub fn start_duration_scan(&mut self, _state_dir: PathBuf) {
+    pub fn start_duration_scan(&mut self) {
         // Signal any running scan to stop.
         self.scan_stop.store(true, Ordering::SeqCst);
         // Create a fresh stop flag for any new scan cycle.