ui: the STK menu button shows the title without the "STK: " prefix

The Simulator view's "STK menu" block already carries the legend, so the
emerald button's hardcoded "STK: " prefix was redundant.  It now shows just
the card's menu title (e.g. "Alfa Mobile"), or "Menu"/"Меню" for a
title-less SET UP MENU (`stkMenuLabel`, unchanged).

The help pages (EN/RU) describe the button as the menu title, both AGENTS
files drop the old `STK: Menu` wording, and stk_menu.test.js gains a source
guard against the prefix returning.  sw cache simple-v341.
Tests: 743 Node / 619 Python.

frontend/index.html

@@ -1152,7 +1152,7 @@
 				<fieldset class="border border-gray-200 dark:border-slate-700 rounded p-3">
 					<legend class="px-1 text-xs font-medium text-gray-500 dark:text-slate-400" data-l10n="STK menu">STK menu</legend>
 					<div class="flex items-center gap-2">
-						<button id="stk-menu-btn" data-needs="card" onclick="stkMenuOpen()" class="px-2.5 py-1 text-sm rounded bg-emerald-600 text-white hover:bg-emerald-700 disabled:opacity-40 disabled:cursor-not-allowed" style="display:none">STK: <span id="stk-menu-title"></span></button>
+						<button id="stk-menu-btn" data-needs="card" onclick="stkMenuOpen()" class="px-2.5 py-1 text-sm rounded bg-emerald-600 text-white hover:bg-emerald-700 disabled:opacity-40 disabled:cursor-not-allowed" style="display:none"><span id="stk-menu-title"></span></button>
 						<span id="stk-menu-none" class="text-xs text-gray-500 dark:text-slate-400" style="display:none" data-l10n="No menu set by the card">No menu set by the card</span>
 					</div>
 				</fieldset>