fix: use per-video caption cache dir to prevent cross-video caption bleed

Subtitle downloads were going to a shared /tmp/video-clipper/subtitles/ dir,
so find_vtt_file could return captions from a previously-loaded video. Now
uses cache_manager::caption_cache_dir (URL-hash-keyed) with pre-download
cleanup, matching the existing per-video caching pattern.

Bump to v0.2.2.

Co-authored-by: Cursor <cursoragent@cursor.com>

src-tauri/tauri.conf.json

@@ -1,7 +1,7 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
   "productName": "GUI Video Clipper",
-  "version": "0.2.1",
+  "version": "0.2.2",
   "identifier": "xyz.cottongin.gui-video-clipper",
   "build": {
     "beforeDevCommand": "npm run dev",