[Personal-WP] Add GitHub OAuth integration for private repositories

[akirk]

Motivation for the change, related issues

This PR adds GitHub OAuth support to personal-wp, allowing blueprints to access private GitHub repositories. This mirrors the existing OAuth support in the main website.

Todo after merging

Create a new Github app and configure the CLIENT_ID and CLIENT_SECRET for this deployment.

Contains fixes from #3181

Implementation details

OAuth Flow

• Added oauth.php for token exchange with GitHub
• Added GitHubPrivateRepoAuthModal component to prompt for authentication when a blueprint requires private repo access
• createGitAuthHeaders() provides authentication headers for GitHub API requests

Blueprint Preservation

• Hash fragment blueprints are converted to blueprint-url query parameters early in main.tsx to survive the OAuth redirect flow
• buildOAuthRedirectUrl() also handles this conversion when building the redirect URL

Multi-tab Handling

• When a new tab opens with a blueprint requiring OAuth, it properly coordinates with existing tabs via the takeover mechanism
• Key fix: Token is stored in sessionStorage (not just memory) to survive the takeover reload. sessionStorage is cleared when the tab closes, maintaining security.

Error Handling

• Added detection for GitAuthenticationError in the boot error handler
• Added early return in catch block to prevent URL clearing after errors (was stripping blueprint-url before OAuth redirect could preserve it)
• Token capture timing fixed to happen at call time, not creation time

Testing Instructions (or ideally a Blueprint)

1. Deploy personal-wp with oauth.php configured for your GitHub OAuth app
2. Open personal-wp with a blueprint referencing a private GitHub repository:

   https://your-personal-wp.example/#{"steps":[{"step":"installPlugin","pluginData":{"resource":"url","url":"https://github.com/your-username/private-repo/archive/refs/heads/main.zip"}}]}
   

3. Verify the GitHub auth modal appears
4. Complete the OAuth flow
5. Verify the blueprint is applied with the private repository content

Multi-tab test:

1. Open personal-wp in Tab A (main tab)
2. Open a new tab (Tab B) with a blueprint requiring OAuth
3. Tab B should request takeover and show the auth modal
4. Complete OAuth in Tab B
5. Verify Tab B reloads and applies the blueprint (token survives reload via sessionStorage)