fix: SPA routing and prompts table handling

- Add clean URL routing: /dashboard -> dashboard.html, /settings -> settings.html
- Add EnsurePromptsTable for dev environments where prompts table may not exist
- Fix nav links to use clean URLs instead of .html paths

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/static/common.js

@@ -176,8 +176,8 @@ Theme.init();
 function renderNavbar(activePage) {
     const pages = [
         { id: 'builder', label: '构建器', href: '/' },
-        { id: 'dashboard', label: '复盘看板', href: '/dashboard.html' },
-        { id: 'settings', label: '设置', href: '/settings.html' },
+        { id: 'dashboard', label: '复盘看板', href: '/dashboard' },
+        { id: 'settings', label: '设置', href: '/settings' },
     ];
 
     const nav = document.createElement('nav');