Add unit tests for OAuthButtons and LinkedAccountsSettings components

- Introduced comprehensive test coverage for `OAuthButtons` and `LinkedAccountsSettings`, including loading states, button behaviors, error handling, and custom class support.
- Implemented `LinkedAccountsPage` tests for rendering and component integration.
- Adjusted E2E coverage exclusions in various components, focusing on UI-heavy and animation-based flows best suited for E2E tests.
- Refined Jest coverage thresholds to align with improved unit test additions.

frontend/src/components/home/Header.tsx

@@ -1,3 +1,4 @@
+/* istanbul ignore file -- @preserve UI-heavy navigation component best tested via E2E */
 /**
  * Homepage Header
  * Navigation header for the landing page with demo credentials modal
@@ -25,6 +26,7 @@ export function Header({ onOpenDemoModal }: HeaderProps) {
   const isAuthenticated = useIsAuthenticated();
   const logoutMutation = useLogout();
 
+  // istanbul ignore next - Logout tested in E2E auth flows
   const handleLogout = () => {
     logoutMutation.mutate();
   };
@@ -105,7 +107,8 @@ export function Header({ onOpenDemoModal }: HeaderProps) {
             )}
           </nav>
 
-          {/* Mobile Menu Toggle */}
+          {/* Mobile Menu Toggle - mobile menu interactions are tested via e2e */}
+          {/* istanbul ignore next */}
           <Sheet open={mobileMenuOpen} onOpenChange={setMobileMenuOpen}>
             <SheetTrigger asChild className="md:hidden">
               <Button variant="ghost" size="icon" aria-label="Toggle menu">