feat(auth): add change password API and frontend page

Backend:
- Add ChangePasswordReq DTO with validation (current + new password)
- Add AuthService::change_password() method with credential verification,
  password rehash, and token revocation
- Add POST /api/v1/auth/change-password endpoint with utoipa annotation

Frontend:
- Add changePassword() API function in auth.ts
- Add ChangePassword.tsx page with form validation and confirmation
- Add "修改密码" tab in Settings page

After password change, all refresh tokens are revoked and the user
is redirected to the login page.

apps/web/src/api/auth.ts

@@ -51,3 +51,13 @@ export async function refresh(refreshToken: string): Promise<LoginResponse> {
 export async function logout(): Promise<void> {
   await client.post('/auth/logout');
 }
+
+export async function changePassword(
+  currentPassword: string,
+  newPassword: string
+): Promise<void> {
+  await client.post('/auth/change-password', {
+    current_password: currentPassword,
+    new_password: newPassword,
+  });
+}