feat: Add theme system with visual editor

- Backend: themes table and API endpoints (CRUD, export, design-tokens)
- Theme store with preview, apply, and persistence
- ThemesPage with collapsible variables editor and live preview
- Components: ColorPicker (HSL), VariableEditor, ThemePreview, ThemeListItem
- Integration: $theme helper for dynamic components, get_design_tokens MCP tool
- Navigation: /themes route with palette icon in toolbar

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/router/index.ts

@@ -0,0 +1,24 @@
+import { createRouter, createWebHistory } from 'vue-router'
+
+const router = createRouter({
+  history: createWebHistory(),
+  routes: [
+    {
+      path: '/',
+      name: 'canvas',
+      component: () => import('../pages/CanvasPage.vue')
+    },
+    {
+      path: '/components',
+      name: 'components',
+      component: () => import('../pages/ComponentsPage.vue')
+    },
+    {
+      path: '/themes',
+      name: 'themes',
+      component: () => import('../pages/ThemesPage.vue')
+    }
+  ]
+})
+
+export default router