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/SidebarTimer.vue

@@ -1,15 +1,18 @@
 <template>
     <button
-        class="flex w-full items-center justify-center gap-2 rounded-md px-4 py-2 text-sm font-semibold text-white transition"
-        :class="timerStore.isRunning
-            ? 'bg-[#F18619] hover:bg-[#d97314]'
-            : 'bg-primary-500 hover:bg-primary-600'"
+        class="flex w-full items-center justify-center gap-2 rounded-md py-2 text-sm font-semibold text-white transition"
+        :class="[
+            timerStore.isRunning
+                ? 'bg-[#F18619] hover:bg-[#d97314]'
+                : 'bg-primary-500 hover:bg-primary-600',
+            collapsed ? 'px-2' : 'px-4'
+        ]"
         :title="timerStore.isRunning ? 'Arrêter le timer' : 'Démarrer un timer'"
         @click="timerStore.isRunning ? timerStore.stop() : timerStore.start()"
     >
         <Icon
             :name="timerStore.isRunning ? 'mdi:stop' : 'mdi:play'"
-            size="16"
+            :size="collapsed ? '20' : '16'"
         />
         <span v-if="!collapsed" class="font-mono tracking-wide">
             {{ timerStore.elapsedFormatted }}