feat: implement Google OAuth authentication

- Add Google OAuth 2.0 login flow with passport-google-oauth20
- Create User and RefreshToken entities for session management
- Implement JWT access tokens (15min) + HttpOnly refresh cookies (7 days)
- Add auth endpoints: /google, /google/callback, /refresh, /me, /logout
- Create LoginPage with Google sign-in button (shadcn/ui)
- Add AuthGuard for protected routes with redirect preservation
- Implement silent token refresh on app mount
- Add UserMenu component with avatar and sign-out

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

backend/src/modules/auth/dto/auth-response.dto.ts

@@ -0,0 +1,28 @@
+import { IsString, IsUUID, IsEmail, IsOptional, IsUrl } from 'class-validator';
+
+export class UserResponseDto {
+  @IsUUID()
+  id: string;
+
+  @IsEmail()
+  email: string;
+
+  @IsString()
+  displayName: string;
+
+  @IsOptional()
+  @IsUrl()
+  avatarUrl?: string;
+}
+
+export class AuthResponseDto {
+  @IsString()
+  accessToken: string;
+
+  user: UserResponseDto;
+}
+
+export class MessageResponseDto {
+  @IsString()
+  message: string;
+}