ci(a11y): cut the IBM Equal Access scan from ~73 min to ~13 min per PR (#14633)

* ci(a11y): shard the IBM Equal Access scan job across 8 runners

The scan step ran ~200 tests sequentially on one runner (--workers=1,
--retries=2), taking ~70 min per PR. Split it into 8 shards with 2
workers each and no retries, merge the per-shard report artifacts in a
summary job, cancel superseded PR runs, and only trigger on frontend
changes.

* test(e2e): seed the starter flow at desktop width regardless of test viewport

Each CI shard starts from a fresh DB, so a mobile-viewport a11y test can be
the first to hit the empty workspace. Seeding drives the desktop flow editor
(templates modal -> component sidebar), which is unreachable at 390px, so
widen just for the seed and restore the test's viewport afterwards.

Also allow one Playwright retry per shard: a retry only costs time when a
spec fails, while a red check from a flake costs a whole re-run.

* ci(a11y): read the dispatch ref via env and validate it; resize inside try

Address CodeRabbit review: pass inputs.ref through the step environment
instead of interpolating it into the script, reject malformed refs before
they reach GITHUB_OUTPUT, and move the seed-viewport resize inside the
existing try so the finally cleanup always runs.