server: guarantee a TERMINAL RESPONSE for paused proactive commands

Every FETCHed proactive command must be answered, otherwise the card is
left in an unfinished session and stops issuing commands (e.g. it will
not deliver a PoR for SEND SM). The menu handlers now share
_menu_send_response, and a server-side watchdog (_arm_menu_timeout /
_menu_timeout_fire, --menu-timeout, default 60s, 0 disables) sends the
timeout result (0x12) when the user never answers. The timer is armed
while a command is pending and cancelled on any response, equip, rescue
and card disconnect.

Also fixes docs/api.md, which had back (0x11) and timeout (0x12) codes
swapped. Tests for arm/cancel/clamping and the flat timeout TR. SW cache
v89 -> v90.

frontend/help.html

@@ -336,7 +336,7 @@
 	<p class="text-sm mb-3">Interacts with the Card Application Toolkit session: the STK menu, subscribed events, the proactive command log, the PROVIDE LOCAL INFORMATION data dictionary, and STATUS polling.</p>
 
 	<h4 id="stk-menu" class="font-medium mb-1">5.5.1 STK menu</h4>
-	<p class="text-sm mb-3">When the card has issued a SET UP MENU command, a &ldquo;STK menu&rdquo; block appears at the top of this view with an emerald <strong>STK: &lt;title&gt;</strong> button that opens the menu overlay (same as the card&rsquo;s STK menu browser). If the card has not set up a menu, the block shows &ldquo;No menu set by the card&rdquo; instead. The menu state is refreshed each time the view is opened.</p>
+	<p class="text-sm mb-3">When the card has issued a SET UP MENU command, a &ldquo;STK menu&rdquo; block appears at the top of this view with an emerald <strong>STK: &lt;title&gt;</strong> button that opens the menu overlay (same as the card&rsquo;s STK menu browser). If the card has not set up a menu, the block shows &ldquo;No menu set by the card&rdquo; instead. The menu state is refreshed each time the view is opened. User-interactive proactive commands always get a TERMINAL RESPONSE: the overlay pauses for your choice, and if you neither answer nor press <strong>Timeout</strong>, the server answers with a timeout result after the <code class="font-mono text-sm">--menu-timeout</code> seconds (default 60, <code class="font-mono text-sm">0</code> disables).</p>
 
 	<h4 id="subscribed-events" class="font-medium mb-1">5.5.2 Subscribed events (SET UP EVENT LIST)</h4>
 	<p class="text-sm mb-2">The events the card monitors. Each event has a <strong>Send</strong> button that opens a form specific to the event type:</p>