fix: add error-state UI with retry button for failed OAuth callback (Closes #493) [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Summary

Replaces the passive 3-second auto-redirect with an actionable retry button on the OAuth callback error state, so users can immediately retry sign-in instead of waiting.

Changes

AuthCallbackPage.tsx

• Replaced the auto-redirect setTimeout with a clickable "Try signing in again" button
• Removed the "Redirecting to sign in..." text
• Applies to all three error paths: error param, missing token, and login failure

SignInPage.test.tsx

• Updated 3 existing tests that relied on vi.advanceTimersByTime(3000) to click the retry button instead
• Added 5 new tests in a AuthCallbackPage retry button describe block covering error param, network failure, missing token, navigation, and dark theme

Acceptance Criteria

• A denied-consent or backend-failure callback shows a clear error state with a retry option, not an infinite spinner
• Navigating back to this page after a completed auth doesn't get stuck
• The happy-path success flow is unaffected

Closes #493