feat: add SMTP settings page and user management features

- Added a new SMTP settings page with functionality to configure SMTP settings, test connections, and send test emails.
- Implemented user management page to list users, invite new users, and manage user permissions.
- Created modals for inviting users and editing user permissions.
- Added tests for the new SMTP settings and user management functionalities.
- Updated navigation to include links to the new SMTP settings and user management pages.

frontend/src/api/smtp.ts

@@ -0,0 +1,50 @@
+import client from './client'
+
+export interface SMTPConfig {
+  host: string
+  port: number
+  username: string
+  password: string
+  from_address: string
+  encryption: 'none' | 'ssl' | 'starttls'
+  configured: boolean
+}
+
+export interface SMTPConfigRequest {
+  host: string
+  port: number
+  username: string
+  password: string
+  from_address: string
+  encryption: 'none' | 'ssl' | 'starttls'
+}
+
+export interface TestEmailRequest {
+  to: string
+}
+
+export interface SMTPTestResult {
+  success: boolean
+  message?: string
+  error?: string
+}
+
+export const getSMTPConfig = async (): Promise<SMTPConfig> => {
+  const response = await client.get<SMTPConfig>('/settings/smtp')
+  return response.data
+}
+
+export const updateSMTPConfig = async (config: SMTPConfigRequest): Promise<{ message: string }> => {
+  const response = await client.post<{ message: string }>('/settings/smtp', config)
+  return response.data
+}
+
+export const testSMTPConnection = async (): Promise<SMTPTestResult> => {
+  const response = await client.post<SMTPTestResult>('/settings/smtp/test')
+  return response.data
+}
+
+export const sendTestEmail = async (request: TestEmailRequest): Promise<SMTPTestResult> => {
+  const response = await client.post<SMTPTestResult>('/settings/smtp/test-email', request)
+  return response.data
+}