ui: upgrade to FontAwesome 6, harden burger-menu shim

- Drop FA4 font files and font-awesome.min.css.
- Ship FA6 webfonts (solid, regular, brands) and fa6.min.css.
- io-indicator: use FA6 names (fa-circle-plus / -minus / -exclamation).
- static/js/ui.js: no-op the legacy side-menu click handler when menu
  links are not present (V09 chrome removes them) so the Settings tab
  no longer logs 'cannot set properties of null'.

src/js/io-indicator.js

@@ -49,14 +49,17 @@ module.exports = {
     methods: {
         get_io_state_class: function(active, state) {
             if (typeof active == "undefined" || typeof state == "undefined") {
-                return "fa-exclamation-triangle warn";
+                return "fa-triangle-exclamation warn";
             }
 
+            // Tristated: render as the regular (outline) circle to
+            // distinguish from active/inactive solid circles. Adding
+            // `far` switches to the FA6 regular family.
             if (state == 2) {
-                return "fa-circle-o";
+                return "far fa-circle";
             }
 
-            const icon = state ? "fa-plus-circle" : "fa-minus-circle";
+            const icon = state ? "fa-circle-plus" : "fa-circle-minus";
             return `${icon} ${active ? "active" : "inactive"}`;
         },