feat: Frontend JWT token management and login page

- Created useAuthApi composable for JWT token lifecycle management
- Implemented setupAuthInterceptor for automatic Authorization header injection
- Added LoginPage.vue with username/password form
- Configured router to redirect to /login for unauthenticated access
- Token stored in localStorage with expiration tracking
- Automatic token validation and cleanup on expiration
- All fetch requests automatically include Bearer token
- Unit tests for login, logout, token validation flows

This enables frontend to authenticate via JWT tokens in production mode.

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

frontend/src/app/router.ts

@@ -3,7 +3,8 @@ import { createRouter, createWebHistory } from 'vue-router'
 export const router = createRouter({
   history: createWebHistory(),
   routes: [
-    { path: '/', redirect: '/home' },
+    { path: '/', redirect: '/login' },
+    { path: '/login', component: () => import('../features/auth/pages/LoginPage.vue'), meta: { title: 'Login' } },
     { path: '/home', component: () => import('../features/home/pages/HomePage.vue'), meta: { screenId: 'SCR-000', templateId: 'T00', module: 'Home', section: 'Home', title: '홈', order: 0, favoriteAllowed: false } },
     { path: '/research/sell-decision', component: () => import('../features/sell-decision/pages/SellDecisionPage.vue'), meta: { screenId: 'SCR-002', templateId: 'T03', module: 'Research', section: 'Research', title: '매도 의사결정', order: 1, favoriteAllowed: true } },
     { path: '/ops/data-quality', component: () => import('../features/data-quality/pages/DataQualityPage.vue'), meta: { screenId: 'SCR-013', templateId: 'T08', module: 'Operations', section: 'Operations', title: '데이터 품질', order: 1, favoriteAllowed: true } },