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/pages/projects/[id]/groups.vue

@@ -1,10 +1,12 @@
 <template>
     <div>
-        <div class="flex items-center justify-between">
-            <h1 class="text-2xl font-bold text-primary-500">{{ project?.name ?? '' }} — Groupes</h1>
+        <div class="sticky top-8 z-20 bg-white pb-4 sm:top-12">
+            <div class="flex items-center justify-between">
+                <h1 class="text-xl font-bold text-primary-500 sm:text-2xl">{{ project?.name ?? '' }} — Groupes</h1>
+            </div>
         </div>
 
-        <div class="mt-6">
+        <div>
             <ProjectGroupTab :project-id="projectId" />
         </div>
     </div>