fix(auth): show OAuth callback errors

[AyobamiH]

Summary

• Show a dismissible alert when the OAuth callback returns an error
• Map the known auth_error code to clear, user-friendly copy
• Use a Map so every unknown URL value, including prototype property names such as proto, safely falls back to generic copy
• Add focused tests for rendering, dismissal, adversarial unknown-code handling, and the no-error state
• Give the dismiss control a 24px pointer target

Fixes #162.

Verification

• npx vitest run src/app/login/login-form.test.tsx (3 passed)
• npm run test:unit (131 passed)
• npm run lint
• npx tsc --noEmit
• npm run build
• git diff --check
• npm run dev reached ready successfully

The visible alert and dismissal behaviour are covered by the repository’s jsdom component tests.

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