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/MonitorPage.tsx

@@ -110,7 +110,7 @@ export default function MonitorPage() {
       <div className="stats-grid">
         <div className="stat-card total">
           <span className="stat-number">{data.issues.total_issues}</span>
-          <span className="stat-label">Total Issues</span>
+          <span className="stat-label">Total Tasks</span>
         </div>
         <div className="stat-card" style={{ borderLeftColor: 'var(--accent)' }}>
           <span className="stat-number">{data.issues.new_issues_24h}</span>