perf(thumbnails): scaled JPEG decode and continuous worker batching

Major speedups for large-folder thumbnail generation:

- JPEGs decode through mozjpeg at the smallest DCT scale covering
  320px instead of full resolution, with catch_unwind and a fallback
  to the image crate for anything mozjpeg rejects. EXIF orientation
  preserved in both paths.
- RGB8 pipeline end to end (no RGBA round-trip) and CatmullRom resize
  in place of Lanczos3.
- Video posters grab the seeked frame directly instead of running the
  ffmpeg thumbnail filter (which decoded ~100 frames), with decode
  threads capped at 2 per process.
- Workers only sleep when the queue is empty rather than 250ms after
  every batch. Image batches commit before videos start, and videos
  run sequentially off the rayon pool (blocking ffmpeg waits were
  starving image decode) with per-item commits so progress keeps
  moving through slow stretches.

src-tauri/Cargo.toml

@@ -53,6 +53,7 @@ csv = "1"
 kamadak-exif = "0.5"
 notify = "6"
 tauri-plugin-notification = "2"
+mozjpeg = "0.10.13"
 
 # ── Dev-mode performance ────────────────────────────────────────────────────
 # opt-level=1 on the main crate keeps incremental compile short.
@@ -81,6 +82,10 @@ opt-level = 3
 opt-level = 3
 [profile.dev.package.fast_image_resize]
 opt-level = 3
+[profile.dev.package.mozjpeg]
+opt-level = 3
+[profile.dev.package.mozjpeg-sys]
+opt-level = 3
 
 # Parallel work scheduler
 [profile.dev.package.rayon]