fix(#411): resolve CI lint errors — prettier, unused directives, no-base-to-string

- auth.config.ts: collapse multiline template literal to single line
- auth.controller.ts: add eslint-disable for intentional no-unnecessary-condition
- auth.service.ts: remove 5 unused eslint-disable directives (Node 24 resolves
  BetterAuth types), fix prettier formatting, fix no-base-to-string
- login/page.tsx: remove unnecessary String() wrapper
- auth-context.test.tsx: fix prettier line length

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/app/(auth)/login/page.tsx

@@ -103,7 +103,7 @@ export default function LoginPage(): ReactElement {
 
         if (result.error) {
           const parsed = parseAuthError(
-            result.error.message ? new Error(String(result.error.message)) : result.error
+            result.error.message ? new Error(result.error.message) : result.error
           );
           setError(parsed.message);
         } else {

apps/web/src/lib/auth/auth-context.test.tsx

@@ -330,7 +330,9 @@ describe("AuthContext", (): void => {
 
       // An Error that doesn't match any known pattern (parseAuthError returns "unknown")
       // should fall through to the instanceof Error catch-all and return "backend"
-      vi.mocked(apiGet).mockRejectedValueOnce(new Error("Something completely unexpected happened"));
+      vi.mocked(apiGet).mockRejectedValueOnce(
+        new Error("Something completely unexpected happened")
+      );
 
       render(
         <AuthProvider>