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/projects/[id]/index.vue

@@ -298,7 +298,10 @@ const filteredTasks = computed(() => {
         result = result.filter(t => t.tags?.some(tag => tag.id === selectedTagId.value))
     }
     if (selectedAssigneeId.value) {
-        result = result.filter(t => t.assignee?.id === selectedAssigneeId.value)
+        result = result.filter(t =>
+            t.assignee?.id === selectedAssigneeId.value
+            || t.collaborators?.some(c => c.id === selectedAssigneeId.value)
+        )
     }
     if (selectedStatusId.value) {
         result = result.filter(t => t.status?.id === selectedStatusId.value)