v3.11.0 editor polish: keyset editor as a header table, in sync with TAR

The keyset rows were a stack of flex divs with no column labels.  They are
now a fixed-layout table like the TAR table: a thead with # (keyset number,
with its tooltip), KIc, KID, KIc key, KID key, Counter and the action
column, and <tr> rows of py-1 px-2 cells with w-full inputs (the columns
drive the widths: 40/56/56 px, the two key columns split the remainder,
112 px for the 10-digit counter).  Ids and behaviour are unchanged (the
number span still toggles its red flag, the inputs keep their handlers).

Guards: html.test.js checks the keyset table headers and tbody;
cards_form.test.js checks the row markup.  sw cache simple-v366.

Tests: 665 Python + 788 frontend green; the whole inline PWA script
syntax-checked.

frontend/tests/html.test.js

@@ -152,6 +152,12 @@ test('the card editor groups its fields into labelled fieldsets', () => {
     // the keyset editor and the PSK pair stay as implemented
     assert.match(scp80, /id="cards-keysets"/);
     assert.match(scp80, /onclick="cardsKeysetAdd\(\)"/);
+    // ... and mirror the TAR table: a fixed-layout table with headers
+    assert.match(scp80, /<table class="w-full table-fixed text-sm mb-2">/);
+    assert.match(scp80, /<tbody id="cards-keysets">/);
+    for (const h of ['KIc', 'KID', 'KIc key', 'KID key', 'Counter']) {
+        assert.match(scp80, new RegExp('>' + h + '</th>'));
+    }
     assert.match(scp81, /id="cards-psk-id"/);
     assert.match(scp81, /id="cards-psk-key"/);
     // the PSK explanation lives inside the SCP81 group, not outside it