feat(time-tracking) : add calendar page, timer sidebar, and all UI components

- SidebarTimer widget with play/stop button
- TimeEntryBlock with drag-to-move and resize
- TimeEntryDrawer for create/edit entries
- TimeEntryContextMenu for copy/paste/delete
- TimeTrackingCalendar grid with week/day view
- Time tracking page with filters and navigation
- Sidebar link and timer integration in layout
- TaskCard play button connected to timer store

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

frontend/components/SidebarTimer.vue

@@ -0,0 +1,33 @@
+<template>
+    <div class="flex items-center gap-2">
+        <button
+            class="flex items-center justify-center rounded-full transition-colors"
+            :class="timerStore.isRunning
+                ? 'bg-red-500 hover:bg-red-600 text-white'
+                : 'bg-green-500 hover:bg-green-600 text-white'"
+            style="width: 32px; height: 32px;"
+            :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="18"
+            />
+        </button>
+        <span
+            v-if="!collapsed"
+            class="font-mono text-sm font-bold"
+            :class="timerStore.isRunning ? 'text-white' : 'text-neutral-400'"
+        >
+            {{ timerStore.elapsedFormatted }}
+        </span>
+    </div>
+</template>
+
+<script setup lang="ts">
+defineProps<{
+    collapsed: boolean
+}>()
+
+const timerStore = useTimerStore()
+</script>