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/App.svelte

@@ -1285,7 +1285,8 @@
           if (azureOrganization) azureOrganization.tokenStored = false;
           else providerConfig.tokenStored = false;
         } else if (update.token) {
-          const username = (azureOrganization?.username ?? providerConfig.username).trim() || "oauth2";
+          const fallbackUsername = update.provider === "github" ? "x-access-token" : "oauth2";
+          const username = (azureOrganization?.username ?? providerConfig.username).trim() || fallbackUsername;
           await credSave(key, username, update.token, "token");
           if (azureOrganization) azureOrganization.tokenStored = true;
           else providerConfig.tokenStored = true;