fix(dashboard) : appliquer le filtre user aux KPIs et charts de taches

Avant, seul le KPI "Heures sur la periode" reagissait au filtre Utilisateur ;
"Taches totales", "Mes taches actives" et tous les graphiques tache restaient
inchanges. Le computed tasks ne filtrait que par projet, et myTasks etait
hardcode sur auth.user.id (cf ticket LST40).

Ajoute un effectiveUserId (selectedUser ?? auth.user) et applique le filtre
user a tasks pour propager dans tous les charts et KPIs.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/pages/index.vue

@@ -93,11 +93,22 @@ const isWeekPeriod = computed(() =>
     selectedPeriod.value === 'thisWeek' || selectedPeriod.value === 'lastWeek'
 )
 
-// ── Filtered data (client-side project filter) ──
+// ── Filtered data (client-side project + user filter) ──
+
+const effectiveUserId = computed(() => selectedUserId.value ?? auth.user?.id ?? null)
 
 const tasks = computed(() => {
-    if (!selectedProjectId.value) return allTasks.value
-    return allTasks.value.filter(t => t.project?.id === selectedProjectId.value)
+    let result = allTasks.value
+    if (selectedProjectId.value) {
+        result = result.filter(t => t.project?.id === selectedProjectId.value)
+    }
+    if (selectedUserId.value) {
+        result = result.filter(t =>
+            t.assignee?.id === selectedUserId.value
+            || t.collaborators?.some(c => c.id === selectedUserId.value),
+        )
+    }
+    return result
 })
 
 const timeEntries = computed(() => {
@@ -173,8 +184,8 @@ const totalHoursThisWeek = computed(() =>
 
 const myTasks = computed(() =>
     tasks.value.filter(t =>
-        t.assignee?.id === auth.user?.id
-        || t.collaborators?.some(c => c.id === auth.user?.id)
+        t.assignee?.id === effectiveUserId.value
+        || t.collaborators?.some(c => c.id === effectiveUserId.value),
     )
 )