fix(web): stop closed mobile nav from intercepting page clicks

The two-column mobile nav set display:block on #mobile-nav (an ID
selector) to override .menu's display:flex so column-count worked.
That ID specificity also beat DaisyUI's closed-state display:none
rule, so the menu stayed display:block while 'closed' (only opacity
dropped to 0). The invisible, two-column-wide overlay captured clicks
over underlying page content and fired SPA navigation.

Scope display:block (and the column layout) to the dropdown's open
state (.dropdown-open / :focus-within) so DaisyUI's display:none can
hide the menu and its links when closed.

src/meshcore_hub/web/static/css/app.css

@@ -424,7 +424,8 @@
         font-size: 0.9375rem;
     }
 
-    #mobile-nav {
+    .dropdown.dropdown-open #mobile-nav,
+    .dropdown:focus-within #mobile-nav {
         display: block;
         column-count: 2;
         column-gap: 0.25rem;