refactor(frontend) : make page headers and filters sticky across all pages

Wrap title + filters in a sticky container (top-8 sm:top-12, z-20, bg-white)
on all pages for consistent scroll behavior. Also fix SidebarTimer icon
visibility when sidebar is collapsed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/stores/ui.ts

@@ -1,5 +1,6 @@
 export const useUiStore = defineStore('ui', () => {
     const sidebarCollapsed = ref(false)
+    const sidebarOpen = ref(false)
 
     if (import.meta.client) {
         const saved = localStorage.getItem('ui-sidebar-collapsed')
@@ -18,5 +19,13 @@ export const useUiStore = defineStore('ui', () => {
         sidebarCollapsed.value = !sidebarCollapsed.value
     }
 
-    return { sidebarCollapsed, toggleSidebar }
+    function openMobileSidebar() {
+        sidebarOpen.value = true
+    }
+
+    function closeMobileSidebar() {
+        sidebarOpen.value = false
+    }
+
+    return { sidebarCollapsed, sidebarOpen, toggleSidebar, openMobileSidebar, closeMobileSidebar }
 })