Release 2026.7.4

Add - JS error hint in the popup and overlay

• When the page's beacons report JS errors (errc), it is now flagged: the
  popup shows a red "N JS error(s) on this page" line, and the on-page overlay
  adds a red ERR N cell. Nothing is shown when the page is error-free.

Add - per-browser unpacked builds (Firefox)

• npm run build emits clean per-browser builds under build/: build/chrome
  (MV3 service worker) and build/firefox (event-page background), plus a loadable
  .zip for each. Firefox does not run an MV3 background service worker, so it
  cannot load the Chrome-targeted committed manifest directly; load
  build/firefox/manifest.json (or the zip) via about:debugging. The committed
  manifest stays Chrome-clean; the release ships the same per-browser zips. The
  build has no dependencies (own stored-zip writer). Documented in both READMEs.