refactor(frontend) : reorganize components into subdirectories and fix imports

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

frontend/components/ui/SidebarTimer.vue

@@ -0,0 +1,26 @@
+<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-red-500 hover:bg-red-600'
+            : 'bg-primary-500 hover:bg-primary-600'"
+        :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"
+        />
+        <span v-if="!collapsed" class="font-mono tracking-wide">
+            {{ timerStore.elapsedFormatted }}
+        </span>
+    </button>
+</template>
+
+<script setup lang="ts">
+defineProps<{
+    collapsed: boolean
+}>()
+
+const timerStore = useTimerStore()
+</script>