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/components/ui/DataTable.vue

@@ -37,7 +37,7 @@
                             <slot name="actions" :item="item" />
                             <button
                                 v-if="deletable"
-                                class="text-[red-500] hover:text-[red-700]"
+                                class="text-neutral-400 transition-colors hover:text-red-500"
                                 @click.stop="$emit('delete', item)"
                             >
                                 <Icon name="mdi:delete-outline" size="20" />