feat: add support for custom JMAP server endpoints in login and settings

app/admin/settings/page.tsx

@@ -112,6 +112,14 @@ export default function AdminSettingsPage() {
       <SettingsSection title="General">
         <TextSetting label="Application Name" configKey="appName" value={currentValue('appName') as string} source={config.appName?.source} onChange={handleChange} onRevert={handleRevert} />
         <TextSetting label="JMAP Server URL" configKey="jmapServerUrl" value={currentValue('jmapServerUrl') as string} source={config.jmapServerUrl?.source} onChange={handleChange} onRevert={handleRevert} placeholder="https://mail.example.com" />
+        <ToggleSetting label="Allow Custom JMAP Endpoint" description="Show a JMAP server URL field on the login form, allowing users to connect to any JMAP server" configKey="allowCustomJmapEndpoint" value={currentValue('allowCustomJmapEndpoint') as boolean} source={config.allowCustomJmapEndpoint?.source} onChange={handleChange} onRevert={handleRevert} />
+        {!!currentValue('allowCustomJmapEndpoint') && (
+          <div className="px-4 py-2.5 bg-amber-50 dark:bg-amber-950/30 border-l-2 border-amber-400 dark:border-amber-600">
+            <p className="text-xs text-amber-800 dark:text-amber-300 leading-relaxed">
+              <strong>CORS warning:</strong> External JMAP servers must include this domain in their CORS <code className="text-[11px] bg-amber-100 dark:bg-amber-900/50 px-1 py-0.5 rounded">Access-Control-Allow-Origin</code> header, or requests from the browser will be blocked.
+            </p>
+          </div>
+        )}
         <ToggleSetting label="Stalwart Features" description="Enable Stalwart Mail Server-specific features" configKey="stalwartFeaturesEnabled" value={currentValue('stalwartFeaturesEnabled') as boolean} source={config.stalwartFeaturesEnabled?.source} onChange={handleChange} onRevert={handleRevert} />
         <TextSetting label="Stalwart API URL" configKey="stalwartApiUrl" value={currentValue('stalwartApiUrl') as string} source={config.stalwartApiUrl?.source} onChange={handleChange} onRevert={handleRevert} placeholder="https://mail.example.com/api" />
         <ToggleSetting label="Demo Mode" description="Enable demo mode with sample data" configKey="demoMode" value={currentValue('demoMode') as boolean} source={config.demoMode?.source} onChange={handleChange} onRevert={handleRevert} />