Add download controls and device storage breakdown

frontend/opfs.js

@@ -156,10 +156,16 @@
           resolve({ ok: false, fallback: true });
           return;
         }
+        let settled = false;
+        const abort = () => finish({ ok: false, paused: true });
         const finish = (result) => {
+          if (settled) return; settled = true;
+          opts.signal?.removeEventListener('abort', abort);
           worker.terminate();
           resolve(result);
         };
+        if (opts.signal?.aborted) { abort(); return; }
+        opts.signal?.addEventListener('abort', abort, { once: true });
         worker.onmessage = (e) => {
           const m = e.data || {};
           if (m.type === 'done')             finish({ ok: true, sha256: m.sha256 || null, expectedSha: m.expectedSha || null, size: m.size || 0 });