UI styling improvements: dashboard headers and navigation

- Restore blue PageHeader on Dashboard (/app-components)
- Update homepage (/) with subtle header design without blue bar
- Add uniform PageHeader styling to application edit page
- Fix Rapporten link on homepage to point to /reports overview
- Improve header descriptions spacing for better readability

backend/src/services/database/singleton.ts

@@ -0,0 +1,28 @@
+/**
+ * Database Adapter Singleton
+ * 
+ * Provides a shared database adapter instance to prevent multiple connections.
+ * All services should use this singleton instead of creating their own adapters.
+ */
+
+import { createDatabaseAdapter } from './factory.js';
+import type { DatabaseAdapter } from './interface.js';
+
+let dbAdapterInstance: DatabaseAdapter | null = null;
+
+/**
+ * Get the shared database adapter instance
+ */
+export function getDatabaseAdapter(): DatabaseAdapter {
+  if (!dbAdapterInstance) {
+    dbAdapterInstance = createDatabaseAdapter(undefined, undefined, false); // Don't allow close (singleton)
+  }
+  return dbAdapterInstance;
+}
+
+/**
+ * Reset the singleton (for testing only)
+ */
+export function resetDatabaseAdapter(): void {
+  dbAdapterInstance = null;
+}