Fix mobile navbar and misc cleanups (#25134) (#25169)

Backport #25134 by @silverwind

- Fix and improve mobile navbar layout
- Apply all cleanups suggested in
https://github.com/go-gitea/gitea/pull/25111
- Make media query breakpoints match Fomantic's exactly
- Clean up whitespace in class on navbar items

Mobile navbar before and after:
<img width="745" alt="Screenshot 2023-06-08 at 08 40 56"
src="ca84b239-b10f-41db-8c06-dcf2b6dd9d28">
<img width="739" alt="Screenshot 2023-06-08 at 08 41 23"
src="09133c54-eb7e-4110-858c-ead23c3b7521">

web_src/js/features/common-global.js

@@ -20,18 +20,14 @@ export function initGlobalFormDirtyLeaveConfirm() {
 }
 
 export function initHeadNavbarContentToggle() {
-  const content = $('#navbar');
-  const toggle = $('#navbar-expand-toggle');
-  let isExpanded = false;
-  toggle.on('click', () => {
-    isExpanded = !isExpanded;
-    if (isExpanded) {
-      content.addClass('shown');
-      toggle.addClass('active');
-    } else {
-      content.removeClass('shown');
-      toggle.removeClass('active');
-    }
+  const navbar = document.getElementById('navbar');
+  const btn = document.getElementById('navbar-expand-toggle');
+  if (!navbar || !btn) return;
+
+  btn.addEventListener('click', () => {
+    const isExpanded = btn.classList.contains('active');
+    navbar.classList.toggle('navbar-menu-open', !isExpanded);
+    btn.classList.toggle('active', !isExpanded);
   });
 }