fix(repo): prevent GitHub API calls when GitHub is not configured

[jimcavoli]

Summary

This PR fixes issue #625 where Kubero was making calls to GitHub APIs even when GitHub was disabled (no PAT set) and only Gitea was configured.

Problem

On Kubero v2.4.6, when setting up a pipeline with Gitea as the only configured provider:

• Even with GitHub disabled (PAT not set), Kubero was still making calls to GitHub APIs and failing
• Kubero was also making HTTP calls to the GitHub APIs with Gitea repository paths

Root Causes

1. Client-side: The default selected tab (repotab) was hardcoded to 'github' regardless of which providers were actually enabled
2. Server-side: The GitHub API constructor didn't properly handle undefined baseUrl values (the check if (baseUrl === '') missed undefined values)
3. Server-side: All GitHub API methods would try to make HTTP requests even when no token was configured

Changes

Client (client/src/components/pipelines/form.vue)

• Made listRepositories() async and wait for provider list to load before setting default tab
• Set default tab to first enabled provider instead of hardcoding 'github'
• Added guard in loadRepository() to skip API calls if the selected provider is not enabled

Server (server/src/repo/git/github.ts)

• Fixed baseUrl check to handle both empty string and falsy/undefined values
• Added token property and isConfigured() method to check if the API is properly configured
• Added configuration guards to all public API methods:
  • getRepository()
  • getRepositories()
  • listRepos()
  • getBranches()
  • getReferences()
  • getPullrequests()

Testing

To verify this fix:

1. Set up Kubero with only Gitea configured (no GitHub PAT)
2. Create a new pipeline with "Enable Pipeline to build from Source"
3. Confirm that GitHub tab is grayed out and not selected by default
4. Connect a Gitea repository
5. Verify no GitHub API calls are made in the logs

Fixes #625