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/features/auth/composables/__tests__/useAuthApi.test.ts

@@ -0,0 +1,85 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest'
+import { useAuthApi } from '../useAuthApi'
+
+describe('useAuthApi', () => {
+  beforeEach(() => {
+    // Clear localStorage
+    localStorage.clear()
+    vi.clearAllMocks()
+  })
+
+  it('should initialize with no authentication', () => {
+    const { authState } = useAuthApi()
+    expect(authState.value.isAuthenticated).toBe(false)
+    expect(authState.value.token).toBeNull()
+  })
+
+  it('should login successfully', async () => {
+    global.fetch = vi.fn().mockResolvedValueOnce({
+      ok: true,
+      json: async () => ({
+        accessToken: 'test-token',
+        expiresIn: 3600,
+        tokenType: 'Bearer',
+      }),
+    })
+
+    const { login, authState } = useAuthApi()
+    const result = await login('testuser', 'password', 'Admin')
+
+    expect(result).toBe(true)
+    expect(authState.value.token).toBe('test-token')
+    expect(authState.value.isAuthenticated).toBe(true)
+    expect(localStorage.getItem('kartsell_auth_token')).toBe('test-token')
+  })
+
+  it('should handle login failure', async () => {
+    global.fetch = vi.fn().mockResolvedValueOnce({
+      ok: false,
+      status: 401,
+      json: async () => ({ message: 'Invalid credentials' }),
+    })
+
+    const { login, authState, error } = useAuthApi()
+    const result = await login('testuser', 'wrongpassword', 'Admin')
+
+    expect(result).toBe(false)
+    expect(authState.value.isAuthenticated).toBe(false)
+    expect(error.value).toBeTruthy()
+  })
+
+  it('should logout successfully', () => {
+    localStorage.setItem('kartsell_auth_token', 'test-token')
+    localStorage.setItem('kartsell_expires_at', (Date.now() + 3600000).toString())
+
+    const { logout, authState } = useAuthApi()
+    logout()
+
+    expect(authState.value.token).toBeNull()
+    expect(authState.value.isAuthenticated).toBe(false)
+    expect(localStorage.getItem('kartsell_auth_token')).toBeNull()
+  })
+
+  it('should get token if valid', () => {
+    const expiresAt = Date.now() + 3600000 // 1 hour from now
+    localStorage.setItem('kartsell_auth_token', 'test-token')
+    localStorage.setItem('kartsell_expires_at', expiresAt.toString())
+
+    const { getToken } = useAuthApi()
+    const token = getToken()
+
+    expect(token).toBe('test-token')
+  })
+
+  it('should clear token if expired', () => {
+    const expiresAt = Date.now() - 3600000 // 1 hour ago
+    localStorage.setItem('kartsell_auth_token', 'test-token')
+    localStorage.setItem('kartsell_expires_at', expiresAt.toString())
+
+    const { getToken, authState } = useAuthApi()
+    const token = getToken()
+
+    expect(token).toBeNull()
+    expect(authState.value.isAuthenticated).toBe(false)
+  })
+})