[feat] Add GitHub sync and OAuth support

[arian24b]

Implement GitHub synchronization to allow users to backup their todos to a private repository. Includes:

• OAuth2 flow with PKCE for secure token exchange via a Cloudflare Worker.
• GitHub API integration for repository management and file uploads.
• Settings UI for connecting, syncing, and disconnecting GitHub.
• Local session management for GitHub tokens using sessionStorage.

Setup Checklist (Cloudflare + GitHub OAuth)

GitHub OAuth App

• Create OAuth App in GitHub Developer Settings.
• Homepage URL: your public site URL.
• Authorization callback URL: exactly your redirect URL (must match).
• Keep Client ID + Client Secret.

Frontend env (Vite)

• VITE_GITHUB_CLIENT_ID
• VITE_GITHUB_REDIRECT_URI (must match GitHub OAuth app callback)
• VITE_GITHUB_OAUTH_ENDPOINT (e.g. https://yourdomain.com/oauth/github)
• Set ALLOWED_ORIGIN in Cloudflare Worker (example: https://bezanbere.ir).

Cloudflare Worker

• Deploy worker at /oauth/github.
• Set vars:
• GITHUB_CLIENT_ID
• GITHUB_REDIRECT_URI
• Set secret:
• GITHUB_CLIENT_SECRET

[ohmydevops]

Sorry it's late, I'll check and add it tonight.