TutorialVault: complete Tauri v2 port with runtime fixes

Rename from TutorialDock to TutorialVault. Remove legacy Python app
and scripts. Fix video playback, subtitles, metadata display, window
state persistence, and auto-download of ffmpeg/ffprobe on first run.
Bundle fonts via npm instead of runtime download.

src-tauri/capabilities/default.json

@@ -1,7 +1,7 @@
 {
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
-  "description": "Default capabilities for TutorialDock",
+  "description": "Default capabilities for TutorialVault",
   "windows": ["main"],
   "permissions": [
     "core:default",