Implements critical OAuth 2.0 security improvements to protect against authorization code interception attacks and improve provider compatibility:

- Add PKCE (RFC 7636) support with SHA-256 code challenge/verifier
- Fix access token extraction to use standard field instead of Extra()
- Add OAuth error parameter handling (access_denied, invalid_scope, etc.)
- Maintain backward compatibility for non-PKCE flows

This brings the OAuth implementation up to modern security standards as recommended by Auth0, Okta, and the OAuth 2.0 Security Best Current Practice (RFC 8252).

Backend changes:
- Add code_verifier parameter to ExchangeToken with PKCE support
- Use token.AccessToken for better provider compatibility
- Update proto definition with optional code_verifier field

Frontend changes:
- Generate cryptographically secure PKCE parameters
- Include code_challenge in authorization requests
- Handle and display OAuth provider errors gracefully
- Pass code_verifier during token exchange

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