web: add a session timeout field to the Services card

Adds a numeric input next to the syslog settings to read and set the
session timeout without knowing the command. The current value is
reported in information.json, so the field also shows the default the
firmware was built with.

Costs 86 bytes of code (BANK1) and nothing in RAM.

html/index.html

@@ -375,6 +375,8 @@ button.danger{background:var(--bad);color:#fff;border-color:transparent}
             <input class="in" id="sy-sysip" style="flex:1;min-width:0;width:0" data-i18n-p="sy_sysip">
             <span class="small mut" data-i18n="sy_port">Port</span>
             <input class="in" id="sy-sysport" style="width:64px" type="number" min="1" max="65535"></div>
+          <label class="row" style="justify-content:space-between"><span data-i18n="sy_sesstmo">Session timeout (s)</span>
+            <input class="in" id="sy-sesstmo" style="width:80px" type="number" min="1" max="65535"></label>
           <p class="small mut" data-i18n="sy_services_note">Service state reflects the startup config; runtime state is not readable.</p>
         </div>
         <div class="card"><h2 data-i18n="sy_password">Admin password</h2>