Private repo access via OAuth + “bring your own GitHub token” flow #5

Description
Goal: Allow users to analyze private repositories securely.

Scope:

• Add OAuth GitHub login (web flow) with minimal scopes.
• Enable “Bring Your Own Token” input stored only in browser storage.
• Use user token for GitHub API calls instead of server token when present.
• Update UI to indicate which auth mode is active.
• Handle rate limits and permission errors gracefully.

Acceptance criteria:

• User can authenticate and load a private repo by owner/name.
• User can paste a PAT and access private repos without OAuth.
• Tokens are never logged or sent to analytics.
• Clear error messages for missing permissions.

Notes:

• Consider adding a toggle to prefer user token over server token.