feat(frontend) : add date filter component to time-tracking page

Reusable DateFilter component using @vuepic/vue-datepicker with day/week
toggle. Selecting a day switches to day view, selecting a week navigates
the calendar to that week. Includes "Aujourd'hui" and "Cette semaine"
quick shortcuts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/pages/time-tracking.vue

@@ -75,7 +75,7 @@
             </div>
         </div>
 
-        <div class="mt-4 -mb-24 min-h-0 flex-1">
+        <div class="relative z-0 mt-4 -mb-24 min-h-0 flex-1">
             <TimeEntryList
                 v-if="viewMode === 'list'"
                 :entries="filteredEntries"
@@ -192,28 +192,6 @@ const filteredEntries = computed(() => {
     if (selectedTagId.value) {
         result = result.filter((e) => e.tags.some((t) => t.id === selectedTagId.value))
     }
-    if (selectedDateFilter.value) {
-        if (Array.isArray(selectedDateFilter.value)) {
-            const [start, end] = selectedDateFilter.value
-            const startDay = new Date(start)
-            startDay.setHours(0, 0, 0, 0)
-            const endDay = new Date(end)
-            endDay.setHours(23, 59, 59, 999)
-            result = result.filter((e) => {
-                const entryDate = new Date(e.startedAt)
-                return entryDate >= startDay && entryDate <= endDay
-            })
-        } else {
-            const day = new Date(selectedDateFilter.value)
-            day.setHours(0, 0, 0, 0)
-            const nextDay = new Date(day)
-            nextDay.setDate(nextDay.getDate() + 1)
-            result = result.filter((e) => {
-                const entryDate = new Date(e.startedAt)
-                return entryDate >= day && entryDate < nextDay
-            })
-        }
-    }
     return result
 })
 
@@ -367,4 +345,16 @@ watch(viewMode, () => {
 watch(selectedUserId, () => {
     loadEntries()
 })
+
+watch(selectedDateFilter, (val) => {
+    if (!val) return
+    if (Array.isArray(val)) {
+        startDate.value = getMonday(val[0])
+        viewMode.value = 'week'
+    } else {
+        startDate.value = val
+        viewMode.value = 'day'
+    }
+    loadEntries()
+})
 </script>