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/DashboardPage.tsx

@@ -1,6 +1,6 @@
 import { useState, useEffect } from 'react'
 import api from '@/services/api'
-import type { DashboardStats } from '@/types'
+import type { DashboardStats, Task } from '@/types'
 
 export default function DashboardPage() {
   const [stats, setStats] = useState<DashboardStats | null>(null)
@@ -26,7 +26,7 @@ export default function DashboardPage() {
       <div className="stats-grid">
         <div className="stat-card total">
           <span className="stat-number">{stats.total_issues}</span>
-          <span className="stat-label">Total Issues</span>
+          <span className="stat-label">Total Tasks</span>
         </div>
         {Object.entries(stats.by_status || {}).map(([k, v]) => (
           <div className="stat-card" key={k} style={{ borderLeftColor: statusColors[k] || '#ccc' }}>
@@ -52,7 +52,7 @@ export default function DashboardPage() {
       </div>
 
       <div className="section">
-        <h3>Recent Issues</h3>
+        <h3>Recent Tasks</h3>
         <table>
           <thead>
             <tr><th>ID</th><th>Title</th><th>Status</th><th>Priority</th><th>Type</th><th>Subtype</th></tr>
@@ -61,7 +61,7 @@ export default function DashboardPage() {
             {(stats.recent_issues || []).map((i) => (
               <tr key={i.id}>
                 <td>#{i.id}</td>
-                <td><a href={`/issues/${i.id}`}>{i.title}</a></td>
+                <td><a href={`/tasks/${i.id}`}>{i.title}</a></td>
                 <td><span className={`badge status-${i.status}`}>{i.status}</span></td>
                 <td><span className={`badge priority-${i.priority}`}>{i.priority}</span></td>
                 <td>{i.issue_type}</td><td>{i.issue_subtype || "-"}</td>