Replace saved copies that fail to play, reject truncated saves, and re-check the device cache when the app returns to the foreground

frontend/opfs-worker.js

@@ -75,6 +75,12 @@ self.onmessage = async (e) => {
     } finally {
       access.close();
     }
+    // A connection that ends early can close the body cleanly; never commit
+    // a truncated file, it would be badged "cached" yet fail to play.
+    const expected = Number(res.headers.get('content-length'));
+    if (expected > 0 && offset !== expected) {
+      throw new Error(`download cut short (${offset} of ${expected} bytes)`);
+    }
 
     // Finalize: .part → permanent name. Prefer the native rename, but treat
     // ANY move() failure as "unavailable" and fall back to a chunked copy —