refactor: rename Issue → Task throughout frontend

- Rename files: IssuesPage → TasksPage, IssueDetailPage → TaskDetailPage,
  CreateIssuePage → CreateTaskPage
- Rename TypeScript interface: Issue → Task (keep backend field names)
- Update routes: /issues → /tasks, /issues/new → /tasks/new, /issues/:id → /tasks/:id
- Update CSS class names: issue-* → task-*, create-issue → create-task
- Update UI text: 'Issues' → 'Tasks', 'Create Issue' → 'Create Task'
- Keep 'issue' as a task subtype value in TASK_TYPES dropdown
- Keep all backend API endpoint paths unchanged (/issues, /comments, etc.)
- Rename local Task interface in MilestoneDetailPage to MilestoneTask
  to avoid conflict with the global Task type

src/pages/NotificationsPage.tsx

@@ -52,7 +52,7 @@ export default function NotificationsPage() {
             className={`notification-item ${n.is_read ? 'read' : 'unread'}`}
             onClick={() => {
               if (!n.is_read) markRead(n.id)
-              if (n.issue_id) navigate(`/issues/${n.issue_id}`)
+              if (n.issue_id) navigate(`/tasks/${n.issue_id}`)
             }}
           >
             <div className="notification-dot">{n.is_read ? '' : '●'}</div>