feat : include collaborator tasks in dashboard, my-tasks, and project filters

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

frontend/pages/index.vue

@@ -172,7 +172,10 @@ const totalHoursThisWeek = computed(() =>
 )
 
 const myTasks = computed(() =>
-    tasks.value.filter(t => t.assignee?.id === auth.user?.id)
+    tasks.value.filter(t =>
+        t.assignee?.id === auth.user?.id
+        || t.collaborators?.some(c => c.id === auth.user?.id)
+    )
 )
 
 const myTasksDone = computed(() =>