Add i18n, branding, user management, health checks, and cleanup for deployment

- Multi-language support (EN/DE) with i18n engine and language files
- Configurable branding (name, subtitle, logo) in Settings
- Global default language and per-user language preference
- User management router with CRUD endpoints
- Customer status sync on start/stop/restart
- Health check fixes: derive status from container state, remove broken wget healthcheck
- Caddy reverse proxy and dashboard env templates for customer stacks
- Updated README with real hardware specs, prerequisites, and new features
- Removed .claude settings (JWT tokens) and build artifacts from tracking
- Updated .gitignore for .claude/ and Windows artifacts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

static/css/styles.css

@@ -1,5 +1,11 @@
 /* NetBird MSP Appliance - Custom Styles */
 
+/* i18n FOUC prevention */
+body.i18n-loading #login-page,
+body.i18n-loading #app-page {
+    visibility: hidden;
+}
+
 /* Login */
 .login-container {
     min-height: 100vh;
@@ -163,6 +169,15 @@
     letter-spacing: 0.5px;
 }
 
+.navbar-brand img {
+    object-fit: contain;
+}
+
+/* Login logo */
+#login-logo img {
+    object-fit: contain;
+}
+
 /* Card improvements */
 .card {
     border-radius: 10px;