[qoncept96]

Problem

Writing a tag reports failure even when the write succeeds. The message is:

❌ Write failed. Please try again: AbortError: push is canceled due to a new push request

Web NFC allows exactly one pending push per document. writeNFC()
(templates/write_tag.html:45) builds a fresh NDEFReader and calls write() on
every button press:

1. First press starts a push, which waits for a tag to enter the field.
2. Nothing appears to happen, so the user presses again.
3. The second write() aborts the first, and the first promise rejects with
   AbortError.
4. That rejection is what fills the message box — so the UI says "failed" while
   the second push is usually still live and about to succeed.
5. The user presses again. Repeat forever.

The tag frequently does get written; the UI just never says so.

This is client-side only and unrelated to cloud vs LAN — it affects any Android
Chrome user.

Fix

One reusable NDEFReader, one in-flight write tracked in pendingWrite, and a
second press becomes an explicit AbortController cancel rather than a competing
push. AbortError is reported as "cancelled", not "failed". The button doubles as
Cancel while a write is pending, so the state is visible.

Full diff in the PR; the shape is:

let ndefWriter = null;
let pendingWrite = null;

if (pendingWrite) { pendingWrite.abort(); return; }   // explicit cancel
ndefWriter = ndefWriter || new NDEFReader();
pendingWrite = new AbortController();
ndefWriter.write({records: [...]}, {signal: pendingWrite.signal})
  .then(...)
  .catch(e => e?.name === "AbortError" ? cancelled() : failed(e))
  .finally(() => { pendingWrite = null; restoreButton(); });

Testing

Running in production since 2026-08-30, verified by hand on a Pixel with Chrome
over https: first press waits, tapping the tag writes and now reports success;
pressing again mid-wait reports "cancelled" instead of "failed" and leaves the
reader reusable for the next attempt.

Web NFC needs a secure context, so this is untestable over a plain-http LAN URL —
that path is unchanged ('NDEFReader' in window is false and the existing
unsupported-browser message shows).