v3.13.4: Cards editor counter field: explicit heading, placeholder, validation

The keyset table's column is headed "Counter (hex)" with the placeholder
0000000001, and cardsAdd refuses a save whose counter is not 1-10 hex
digits (empty included; whitespace stripped) - the same rule the store
enforces.  The store keeps its import-compatible empty -> 1 default for
API/import callers.

Docs: repo AGENTS (v3.13.4 note).  sw cache simple-v374.
Tests: 667 Python + 797 frontend green.

frontend/tests/html.test.js

@@ -155,8 +155,8 @@ test('the card editor groups its fields into labelled fieldsets', () => {
     // ... 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>'));
+    for (const h of ['KIc', 'KID', 'KIc key', 'KID key', 'Counter (hex)']) {
+        assert.ok(scp80.includes('>' + h + '</th>'), h);
     }
     assert.match(scp81, /id="cards-psk-id"/);
     assert.match(scp81, /id="cards-psk-key"/);