* ci: run frontend coverage in four shards
Coverage shards reached their 18-minute step limit three times on
2026-10-31 with every test passing. Split the run into four shards with
the limits unchanged; the merge job now expects four raw coverage
artifacts, and the vendor checks move to shard 4.
Closes#563
* test(tags): pin the 390px window check to the first page's last row
Reaching the last row loads the next page (D183), and last() then followed
row 200, far outside the window, so the WebKit check failed in 4-5 of 20
runs on 2 CPUs. Pinned to row 100, it passed 90 of 90 across WebKit,
Chromium and Firefox.
Closes#566
Vito
ef97d01dbd