test: add unit tests for all auth backend logic

- Register: edge cases for non-Error throws, error message assertions,
  and verify signUpEmail was called with correct payload
- Forgot-password: empty/missing email, Error/non-Error throws
- Reset-password: load (missing/expired/valid token) and action (short
  password, mismatch, throw, success redirect)
- Account setup: load (unauthorized, returns user), action (redirect,
  falsy create, throws, isRedirect passthrough)
- Email-verification: correct URL construction, sendMail delegation,
  throw propagation
- Hooks: sets/omits locals, passes building/auth to svelteKitHandler
- Fix Svelte 5  warning on setup page by using  with
  one-time guard instead of top-level prop read

src/lib/server/email-verification.spec.ts

@@ -0,0 +1,64 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+
+const { sendMail } = vi.hoisted(() => ({
+	sendMail: vi.fn()
+}));
+
+vi.mock('$env/static/private', () => ({
+	BETTER_AUTH_URL: 'http://localhost:5173'
+}));
+
+vi.mock('./resend', () => ({
+	sendMail
+}));
+
+import { sendVerificationEmail } from './email-verification';
+
+describe('sendVerificationEmail', () => {
+	beforeEach(() => {
+		sendMail.mockReset();
+	});
+
+	it('calls sendMail with correct email and template', async () => {
+		sendMail.mockResolvedValue(undefined);
+
+		await sendVerificationEmail({
+			email: 'jane@example.com',
+			token: 'abc123'
+		});
+
+		expect(sendMail).toHaveBeenCalledWith({
+			to: 'jane@example.com',
+			templateId: 'email-verification-email',
+			data: {
+				target_email_address: 'jane@example.com',
+				verification_link:
+					'http://localhost:5173/api/auth/verify-email?token=abc123&callbackURL=/login?verified=true'
+			}
+		});
+	});
+
+	it('constructs verification URL with the given token', async () => {
+		sendMail.mockResolvedValue(undefined);
+
+		await sendVerificationEmail({
+			email: 'test@example.com',
+			token: 'xyz789'
+		});
+
+		const call = sendMail.mock.calls[0][0];
+		expect(call.data.verification_link).toContain('xyz789');
+		expect(call.data.verification_link).toContain('/login?verified=true');
+	});
+
+	it('throws when sendMail throws', async () => {
+		sendMail.mockRejectedValue(new Error('Resend error'));
+
+		await expect(
+			sendVerificationEmail({
+				email: 'jane@example.com',
+				token: 'abc123'
+			})
+		).rejects.toThrow('Resend error');
+	});
+});