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

README.md

@@ -45,6 +45,7 @@ This webmail client is designed to work seamlessly with [**Stalwart Mail Server*
 
 ### Security & Privacy
 - External content blocked by default
+- Trusted senders list for automatic image loading
 - HTML sanitization with DOMPurify
 - SPF/DKIM/DMARC status indicators
 - No password storage (session-based auth)
@@ -92,12 +93,14 @@ Edit `.env.local` with your settings:
 
 ```env
 # App name displayed in the UI
-NEXT_PUBLIC_APP_NAME=My Webmail
+APP_NAME=My Webmail
 
-# Your JMAP server URL
-NEXT_PUBLIC_JMAP_SERVER_URL=https://mail.example.com
+# Your JMAP server URL (required)
+JMAP_SERVER_URL=https://mail.example.com
 ```
 
+**Note:** These are runtime environment variables, read at request time. This enables Docker deployments to be configured without rebuilding the image. Legacy `NEXT_PUBLIC_*` variables are still supported as fallbacks.
+
 ### Development
 
 ```bash