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/App.tsx

@@ -5,9 +5,9 @@ import Sidebar from '@/components/Sidebar'
 import LoginPage from '@/pages/LoginPage'
 import SetupWizardPage from '@/pages/SetupWizardPage'
 import DashboardPage from '@/pages/DashboardPage'
-import IssuesPage from '@/pages/IssuesPage'
-import IssueDetailPage from '@/pages/IssueDetailPage'
-import CreateIssuePage from '@/pages/CreateIssuePage'
+import TasksPage from '@/pages/TasksPage'
+import TaskDetailPage from '@/pages/TaskDetailPage'
+import CreateTaskPage from '@/pages/CreateTaskPage'
 import ProjectsPage from '@/pages/ProjectsPage'
 import ProjectDetailPage from '@/pages/ProjectDetailPage'
 import MilestonesPage from '@/pages/MilestonesPage'
@@ -85,9 +85,9 @@ export default function App() {
         <main className="main-content">
           <Routes>
             <Route path="/" element={<DashboardPage />} />
-            <Route path="/issues" element={<IssuesPage />} />
-            <Route path="/issues/new" element={<CreateIssuePage />} />
-            <Route path="/issues/:id" element={<IssueDetailPage />} />
+            <Route path="/tasks" element={<TasksPage />} />
+            <Route path="/tasks/new" element={<CreateTaskPage />} />
+            <Route path="/tasks/:id" element={<TaskDetailPage />} />
             <Route path="/projects" element={<ProjectsPage />} />
             <Route path="/projects/:id" element={<ProjectDetailPage />} />
             <Route path="/milestones" element={<MilestonesPage />} />