feat: Add runtime config, trusted senders, JMAP identities, and UI improvements

- Runtime environment variables for Docker-friendly configuration
- Trusted senders list for automatic image loading
- JMAP identities for proper sender address
- Improved email composer readability
- Horizontal scroll for wide HTML emails

app/api/config/route.ts

@@ -0,0 +1,20 @@
+import { NextResponse } from 'next/server';
+
+/**
+ * Runtime configuration endpoint
+ *
+ * This endpoint serves configuration values that can be set at runtime
+ * via environment variables, enabling post-build configuration for
+ * Docker deployments.
+ *
+ * Priority order:
+ * 1. Runtime env vars (APP_NAME, JMAP_SERVER_URL)
+ * 2. Build-time env vars (NEXT_PUBLIC_APP_NAME, NEXT_PUBLIC_JMAP_SERVER_URL)
+ * 3. Default values
+ */
+export async function GET() {
+  return NextResponse.json({
+    appName: process.env.APP_NAME || process.env.NEXT_PUBLIC_APP_NAME || 'Webmail',
+    jmapServerUrl: process.env.JMAP_SERVER_URL || process.env.NEXT_PUBLIC_JMAP_SERVER_URL || '',
+  });
+}