feat: improve mobile responsiveness across UI

Sidenav becomes a full-screen overlay on mobile with slide-in animation.
Dialog goes full-screen on small screens. Settings dialog gets a mobile
nav with hamburger menu. Touch targets and text sizes scaled up for
mobile. Slider gets visual checked/unchecked colors. Sidebar auto-closes
on mobile route navigation. Resize handle hidden on mobile.

app/composables/useSidebar.ts

@@ -37,6 +37,18 @@ export const useSidebar = () => {
         useCookie('sidebar:width').value = sidebarWidth.value.toString();
     };
 
+    // Auto-close sidebar on mobile when navigating
+    if (import.meta.client) {
+        const mobileQuery = window.matchMedia('(max-width: 767px)');
+        const route = useRoute();
+
+        watch(() => route.path, () => {
+            if (mobileQuery.matches && open.value) {
+                open.value = false;
+            }
+        });
+    }
+
     return {
         open: readonly(open),
         toggle,