fix: resolve GitHub OAuth 404 by adding frontend fallback (#821)

[wingo-blue]

Summary

Fix the GitHub OAuth sign-in flow that returns 404 when users click 'Sign in with GitHub'.

Root Cause

The frontend calls /api/auth/github/authorize\ on the backend, which returns 404 (backend is in a separate private repo). The existing fallback redirected to the same 404 endpoint.

Changes

• auth.ts - Add VITE_GITHUB_CLIENT_ID env var fallback, CSRF state generation, sessionStorage verification
• AuthGuard.tsx - Replace broken 404 fallback with proper error message
• GitHubCallbackPage.tsx - OAuth state validation, user-friendly error page, error state with retry
• .env.example - Add VITE_GITHUB_CLIENT_ID placeholder

Closes #821

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