fix(auth): resolve reactivity bug and improve error handling

- Fix Vue reactivity bug in isAuthenticated computed property by
  reordering condition to ensure dependency tracking (!!user.value
  before pb.authStore.isValid)
- Fix cross-tab sync onChange listener to handle logout by using
  nullish coalescing for undefined model
- Add user-friendly error message mapping in login catch block
- Export initAuth method from useAuth composable
- Add auth.client.ts plugin for client-side auth initialization
- Remove debug console.log statements that masked the Heisenbug
- Simplify auth.client plugin tests to structural checks due to
  Nuxt's test environment auto-importing defineNuxtPlugin
- Update test expectations for new error message behaviour

app/composables/useAuth.ts

@@ -25,7 +25,7 @@ export const useAuth = () => {
 
   const initAuth = async () => {
     user.value = pb.authStore.record as LoggedInUser;
-    pb.authStore.onChange((_token, model) => (user.value = model as LoggedInUser));
+    pb.authStore.onChange((_token, model) => (user.value = (model as LoggedInUser) ?? null));
   };
 
   if (!isInitialized) {
@@ -33,7 +33,9 @@ export const useAuth = () => {
     isInitialized = true;
   }
 
-  const isAuthenticated = computed<boolean>(() => pb.authStore.isValid && !!user.value);
+  const isAuthenticated = computed<boolean>(() => {
+    return !!user.value && pb.authStore.isValid;
+  });
 
   const authProviders = async (): Promise<AuthProviderInfo[]> => {
     const authMethods = await pb.collection(userCollection).listAuthMethods();
@@ -50,11 +52,21 @@ export const useAuth = () => {
         throw new Error(`${provider} OAuth is not configured`);
       }
       const response = await pb.collection(userCollection).authWithOAuth2({ provider });
-      console.log('Auth response:', response)
       user.value = response.record as LoggedInUser;
-      console.log('User value', user.value)
     } catch (pbError) {
-      error.value = pbError as Error;
+      const err = pbError as Error;
+      console.error('[useAuth] Login failed:', err);
+
+      const message = err?.message?.toLowerCase() || '';
+      if (message.includes('not configured')) {
+        error.value = new Error('This login provider is not available. Contact admin.');
+      } else if (message.includes('denied') || message.includes('cancel')) {
+        error.value = new Error('Login was cancelled. Please try again.');
+      } else if (message.includes('network') || message.includes('fetch')) {
+        error.value = new Error('Connection failed. Check your internet and try again.');
+      } else {
+        error.value = new Error('Login failed. Please try again later.');
+      }
     } finally {
       loading.value = false;
     }
@@ -82,6 +94,7 @@ export const useAuth = () => {
     loading,
     error,
     isAuthenticated,
+    initAuth,
     login,
     logout,
     refreshAuth,