feat(auth): add login and registration modal functionality

frontend/src/app/app.component.html

@@ -1,7 +1,45 @@
 <div class="min-h-screen flex flex-col">
   <main class="flex-grow">
-    <app-navbar></app-navbar>
+    <app-navbar 
+      (showLogin)="showLoginForm()" 
+      (showRegister)="showRegisterForm()"
+    ></app-navbar>
     <router-outlet></router-outlet>
   </main>
   <app-footer></app-footer>
+
+  <!-- Auth Forms Overlay -->
+  @if (showLogin() || showRegister()) {
+    <div 
+      class="fixed inset-0 bg-black/50 z-40" 
+      (click)="hideAuthForms()"
+      (keydown.enter)="hideAuthForms()"
+      tabindex="0"
+      role="dialog"
+      aria-modal="true"
+    ></div>
+    <div 
+      class="fixed inset-0 flex items-center justify-center z-50 p-4"
+      role="presentation"
+    >
+      <div 
+        class="relative"
+        role="dialog"
+        aria-modal="true"
+      >
+        @if (showLogin()) {
+          <app-login 
+            (switchForm)="showRegisterForm()"
+            (closeDialog)="hideAuthForms()"
+          ></app-login>
+        }
+        @if (showRegister()) {
+          <app-register 
+            (switchForm)="showLoginForm()"
+            (closeDialog)="hideAuthForms()"
+          ></app-register>
+        }
+      </div>
+    </div>
+  }
 </div>