feat(integrations): support GitHub repositories and enhance repo browser UI

Add GitHub repository support to integrations and update related tests.
Implement server-side GitHub API calls and normalize GitHub base URLs.
Improve the repository browser UI with compact tabs and updated icons.
Add a custom, accessible scrollbar with pointer and keyboard support.

- Add paging, auth headers, and error handling when listing GitHub repos.
- Default GitHub token username to "x-access-token" when saving credentials.
- Introduce compact repo tab styles, new icons, and a draggable scrollbar.

src/lib/components/AppSettingsDialog.svelte

@@ -337,7 +337,7 @@
             <CloudCog size={16} aria-hidden="true" />
             <span>
               <strong>{isGerman ? "Integrationen" : "Integrations"}</strong>
-              <small>GitLab, Azure DevOps & Gitea</small>
+              <small>GitHub, GitLab, Azure DevOps & Gitea</small>
             </span>
             <em>{configuredIntegrationCount(integrationDraft)}</em>
           </button>