test(frontend): improve test coverage and update edge case handling

- Refactor tests to handle empty `model_params` in AgentTypeForm.
- Add return type annotations (`: never`) for throwing functions in ErrorBoundary tests.
- Mock `useAuth` in home page tests for consistent auth state handling.
- Update Header test to validate updated `/dashboard` link.

frontend/tests/components/common/ErrorBoundary.test.tsx

@@ -248,7 +248,7 @@ describe('ErrorBoundary', () => {
 
   describe('error without message', () => {
     it('handles error with null message gracefully', () => {
-      function ThrowNullError() {
+      function ThrowNullError(): never {
         const error = new Error();
         error.message = '';
         throw error;
@@ -338,7 +338,7 @@ describe('ErrorBoundary', () => {
 
   describe('edge cases', () => {
     it('handles deeply nested errors', () => {
-      function DeepChild() {
+      function DeepChild(): never {
         throw new Error('Deep error');
       }
 
@@ -377,7 +377,7 @@ describe('ErrorBoundary', () => {
     });
 
     it('allows nested error boundaries', () => {
-      function InnerThrowing() {
+      function InnerThrowing(): never {
         throw new Error('Inner error');
       }