Fix Google Sign-In broken by Service Worker intercepting auth handler

[quequeo]

Summary

• Fix root cause: Workbox Service Worker navigateFallback was intercepting Firebase reserved /__/auth/handler routes, redirecting them to index.html instead of letting Firebase handle OAuth
• Add navigateFallbackDenylist: [/^\/__\//] to vite.config.js to exclude /__/ paths from SW interception
• Use signInWithPopup in development and signInWithRedirect in production for Google Sign-In
• Default login tab is now "Acceder con Google" instead of "Email y contraseña"
• Extract validateAndReject helper to reduce code duplication in AuthProvider

Test plan

• ESLint: 0 errors
• Prettier: all files pass
• Vitest: 349/349 tests pass (49 test files)
• Pre-push hooks pass
• Local dev tested: Google Sign-In popup works correctly at localhost:5173
• Production: verify /__/auth/handler no longer redirects to /login
• Production: verify Google Sign-In redirect flow works at app.winbit.com.ar