Implement Password Grant Flow for Authentication and Enhance User Experience

- Introduced Password Grant Flow for user authentication, allowing direct login with email and password.
- Updated `useAuth` composable to manage login and logout processes, including Single Sign-Out from Cidaas.
- Enhanced user interface with a new `UserMenu` component displaying user information and logout functionality.
- Updated homepage to show personalized greetings for logged-in users and a login prompt for guests.
- Added logout confirmation page with a countdown redirect to the homepage.
- Documented the implementation details and future enhancements for OAuth2 flows in CLAUDE.md and other relevant documentation.
- Added test credentials and guidelines for automated testing in the new TESTING.md file.

app/components/CommonHeader.vue

@@ -1,13 +1,20 @@
 <script setup lang="ts">
-// experimenta header with branding
+// experimenta header with branding and user menu
+const { loggedIn } = useAuth()
 </script>
 
 <template>
   <header class="header-wrapper">
     <div class="header-content">
+      <!-- Logo (left side) -->
       <NuxtLink to="/" class="logo">
         <img src="/img/experimenta-logo-white.svg" alt="experimenta Logo" class="logo-svg" />
       </NuxtLink>
+
+      <!-- User Menu (right side, only when logged in) -->
+      <div v-if="loggedIn" class="user-menu">
+        <UserMenu />
+      </div>
     </div>
   </header>
 </template>
@@ -26,7 +33,7 @@
   margin: 0 auto;
   padding: 0 20px;
   display: flex;
-  justify-content: center;
+  justify-content: space-between;
   align-items: center;
 }
 
@@ -60,4 +67,9 @@
     width: 200px;
   }
 }
+
+.user-menu {
+  display: flex;
+  align-items: center;
+}
 </style>