feat: add Feishu OAuth integration

Implement Feishu OAuth provider using standard client:
- Set up Feishu-specific endpoints for authorization, token, and userinfo
- Use user_id as sub claim for Feishu user identification
- Extract correct user information from nested 'data' field in Feishu responses

Configuration requirements:
- Set FEISHU_CLIENT_ID and FEISHU_CLIENT_SECRET environment variables to enable Feishu OAuth
- Set ENABLE_OAUTH_SIGNUP=true to allow automatic user creation after OAuth login
- Set DEFAULT_USER_ROLE=user to grant immediate access after OAuth registration
- Set OAUTH_MERGE_ACCOUNTS_BY_EMAIL=true to enable merging of existing user accounts with matching emails

src/routes/auth/+page.svelte

@@ -523,6 +523,16 @@
 											>
 										</button>
 									{/if}
+									{#if $config?.oauth?.providers?.feishu}
+										<button
+											class="flex justify-center items-center bg-gray-700/5 hover:bg-gray-700/10 dark:bg-gray-100/5 dark:hover:bg-gray-100/10 dark:text-gray-300 dark:hover:text-white transition w-full rounded-full font-medium text-sm py-2.5"
+											on:click={() => {
+												window.location.href = `${WEBUI_BASE_URL}/oauth/feishu/login`;
+											}}
+										>
+											<span>{$i18n.t('Continue with {{provider}}', { provider: 'Feishu' })}</span>
+										</button>
+									{/if}
 								</div>
 							{/if}