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/ProjectDetailPage.tsx

@@ -96,7 +96,7 @@ export default function ProjectDetailPage() {
     <div className="project-detail">
       <button className="btn-back" onClick={() => navigate('/projects')}>← Back to projects</button>
 
-      <div className="issue-header">
+      <div className="task-header">
         {editing ? (
           <form className="inline-form" onSubmit={updateProject}>
             <div style={{ fontWeight: 600 }}>{project.name}</div>