Add backend unit test framework and modularize settings UI

Introduced a Vitest-based backend unit testing framework with supporting scripts, helpers, and GitHub Actions integration. Refactored the admin settings page to a modular architecture, splitting monolithic logic into feature-specific tabs and hooks for improved maintainability and testability. Updated documentation to reflect the new testing setup and settings architecture, and added new dependencies for testing utilities.

src/lib/services/auth/OIDCAuthProvider.ts

@@ -82,6 +82,9 @@ export class OIDCAuthProvider implements IAuthProvider {
   async initiateLogin(): Promise<LoginInitiation> {
     try {
       const client = await this.getClient();
+      // Clean up expired states first
+      this.cleanupExpiredStates();
+
       const state = generators.state();
       const nonce = generators.nonce();
       const codeVerifier = generators.codeVerifier();
@@ -95,9 +98,6 @@ export class OIDCAuthProvider implements IAuthProvider {
         timestamp: Date.now(),
       });
 
-      // Clean up expired states
-      this.cleanupExpiredStates();
-
       // Generate authorization URL
       const redirectUrl = client.authorizationUrl({
         scope: 'openid profile email groups',