Run SHA-256 as WebAssembly with JS fallback; note ffmpeg.wasm as deferred

frontend/hash-worker.js

@@ -3,7 +3,7 @@
  * thread, read in 4 MiB slices (never the whole video in memory).
  *
  * In:  { name }                 file name under OPFS videos/ (e.g. "abc.mp4")
- * Out: { ok: true, sha256, size } | { ok: false, error }
+^ * Out: { ok: true, sha256, size, engine } | { ok: false, error }
  * ========================================================================== */
 'use strict';
 importScripts('/sha256.js');
@@ -19,7 +19,7 @@ self.onmessage = async (e) => {
     for (let pos = 0; pos < file.size; pos += STEP) {
       h.update(new Uint8Array(await file.slice(pos, pos + STEP).arrayBuffer()));
     }
-    self.postMessage({ ok: true, sha256: h.hex(), size: file.size });
+    self.postMessage({ ok: true, sha256: h.hex(), size: file.size, engine: self.Sha256.engine() });
   } catch (err) {
     self.postMessage({ ok: false, error: err && err.message ? err.message : String(err) });
   }