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/composables/useCsvExport.ts

@@ -0,0 +1,44 @@
+export type CsvColumn<T> = {
+    header: string
+    value: (row: T) => string | number
+}
+
+/** Escapes a single CSV cell (quotes wrapping + doubled inner quotes). */
+function escapeCell(value: string | number): string {
+    const str = String(value ?? '')
+    if (/[",\n;]/.test(str)) {
+        return `"${str.replace(/"/g, '""')}"`
+    }
+    return str
+}
+
+export function useCsvExport() {
+    /** Builds a CSV string from rows + column definitions (semicolon separator, FR-friendly). */
+    function toCsv<T>(rows: T[], columns: CsvColumn<T>[]): string {
+        const header = columns.map(c => escapeCell(c.header)).join(';')
+        const lines = rows.map(row =>
+            columns.map(c => escapeCell(c.value(row))).join(';'),
+        )
+        return [header, ...lines].join('\n')
+    }
+
+    /** Triggers a browser download of the given CSV content. */
+    function downloadCsv<T>(filename: string, rows: T[], columns: CsvColumn<T>[]): void {
+        if (typeof window === 'undefined') {
+            return
+        }
+        const csv = toCsv(rows, columns)
+        // Prepend BOM so Excel detects UTF-8.
+        const blob = new Blob([`${csv}`], { type: 'text/csv;charset=utf-8;' })
+        const url = URL.createObjectURL(blob)
+        const link = document.createElement('a')
+        link.href = url
+        link.download = filename.endsWith('.csv') ? filename : `${filename}.csv`
+        document.body.appendChild(link)
+        link.click()
+        document.body.removeChild(link)
+        URL.revokeObjectURL(url)
+    }
+
+    return { toCsv, downloadCsv }
+}