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/api/src/auth/auth.controller.ts

@@ -37,6 +37,7 @@ export class AuthController {
     // Defense-in-depth: AuthGuard should guarantee these, but if someone adds
     // a route with AuthenticatedRequest and forgets @UseGuards(AuthGuard),
     // TypeScript types won't help at runtime.
+    // eslint-disable-next-line @typescript-eslint/no-unnecessary-condition
     if (!req.user || !req.session) {
       throw new UnauthorizedException("Missing authentication context");
     }