feat(reporting) : add Reporting dashboard front layer

LST-59 (3.1) front. Completes the Reporting module.

- New frontend/modules/reporting/ layer (auto-detected): /reporting page
  (admin middleware) consuming the 4 read-only report endpoints.
- Filters (period presets + custom dates, project, user). 4 sections (time per
  project, time per user, tasks by status, absences by type) each with a
  DataTable + a Chart.js chart (reused global registration).
- Front-side CSV export per section (useCsvExport: BOM UTF-8, ; separator).
- i18n keys (reporting.*, sidebar.admin.reporting).

nuxt build passes; /reporting routed; no route regression.

frontend/modules/reporting/components/ReportingDoughnut.vue

@@ -0,0 +1,56 @@
+<template>
+    <div class="h-64">
+        <Doughnut
+            v-if="hasData"
+            :data="chartData"
+            :options="options"
+        />
+        <p v-else class="flex h-full items-center justify-center text-sm text-neutral-400">
+            {{ emptyMessage ?? $t('reporting.empty') }}
+        </p>
+    </div>
+</template>
+
+<script setup lang="ts">
+import { Doughnut } from 'vue-chartjs'
+
+const props = defineProps<{
+    labels: string[]
+    values: number[]
+    colors?: string[]
+    emptyMessage?: string
+}>()
+
+const palette = [
+    '#6366f1', '#10b981', '#f59e0b', '#ef4444', '#3b82f6',
+    '#8b5cf6', '#ec4899', '#14b8a6', '#f97316', '#84cc16',
+]
+
+const hasData = computed(() => props.values.some(v => v > 0))
+
+const chartData = computed(() => ({
+    labels: props.labels,
+    datasets: [{
+        data: props.values,
+        backgroundColor: props.colors ?? props.labels.map((_, i) => palette[i % palette.length]),
+        borderWidth: 0,
+    }],
+}))
+
+const options = {
+    responsive: true,
+    maintainAspectRatio: false,
+    cutout: '65%',
+    plugins: {
+        legend: {
+            position: 'bottom' as const,
+            labels: {
+                padding: 16,
+                usePointStyle: true,
+                pointStyle: 'circle',
+                font: { size: 12 },
+            },
+        },
+    },
+}
+</script>