ui: the keyset selector is disabled and cleared while no preset is selected

A keyset only exists inside a card preset, but the SCP80/RAM/TAR keyset
selectors stayed enabled with no preset selected - showing either
"— no keyset —" or a stale "Keyset N — not defined in the preset" entry left
over from an earlier preset.  spKeysetSync() now disables the selector and
reduces it to a "— select a card preset —" placeholder whenever its view has
no preset selected, and re-enables/populates it as soon as one is.

Clearing the preset dropdown ("— Select card —") also refreshed nothing:
cardsApply() returned before the sync, so the stale keyset list stayed; it now
syncs the selectors (and the preset warning) on the empty selection too.

The three selects get disabled:opacity-40 disabled:cursor-not-allowed (the
rules are already in the compiled style.css).  Tests: keysets.test.js covers
the disabled/cleared state and the re-enable plus a cardsApply source guard;
cards_form.test.js and cards_counter.test.js gain tarPresetIdx in their
sandboxes.  Help EN/RU and both AGENTS files; sw cache simple-v343.

frontend/tests/cards_form.test.js

@@ -34,7 +34,7 @@ for (const fn of ['cardsTarValue', 'cardsFormValues', 'cardsClearForm', 'cardsEd
 	'spKeysetOptionsHtml', 'cardsKeysetRowHtml', 'cardsKeysetRowsRender',
 	'cardsKeysetAdd', 'cardsKeysetRemove', 'cardsKeysetKvnUpdate',
 	'cardsKeysetsFromForm', 'spKeysetSync', 'spKeysetApply', 'spKeysetChanged',
-	'ramKeysetChanged', 'ramPresetIdx', 'spKeysetGuard', 'ramKeysetGuard']) {
+	'ramKeysetChanged', 'ramPresetIdx', 'tarPresetIdx', 'spKeysetGuard', 'ramKeysetGuard']) {
 	code += extractFunc(html, fn, ASYNC_FNS.indexOf(fn) >= 0) + '\n';
 }
 code = 'var _cardsKeysetCount = 0;\nglobalThis.spPresetWarningRender = function() {};\n' + code;