Add frontend UI for Connections and linked codebases

[lg-epitech]

Follow up to #22.

Create the frontend UI for linking GitHub accounts and codebases.

User flow:

• User goes to Workspace Settings → Integrations → GitHub.
• User clicks Connect GitHub.
• User installs or authorizes the GitHub App and selects repositories.
• User is redirected back to the app.
• App shows linked repositories.
• User can unlink repositories.

Scope:

• Add a Connections page for linking accounts and codebases.
• Show connection state and linked repositories for the current workspace.
• Support unlinking a repository from the workspace.
• Reflect disconnected or revoked installations gracefully.

This issue is frontend-only. Backend API and authorization work lives in #22.

[linear-code]

DIS-21