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/RepoTabs.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-  import { BookOpen, Database, Plus, X } from "@lucide/svelte";
+  import { Folder, GitBranch, Plus, X } from "@lucide/svelte";
 
   interface RepositoryTabItem {
     path: string;
@@ -28,8 +28,7 @@
     disabled={isBusy}
     title={language === "de" ? "Repository-Verwaltung" : "Repository Management"}
   >
-    <BookOpen size={14} aria-hidden="true" />
-    <span>{language === "de" ? "Repository-Verwaltung" : "Repository Management"}</span>
+    <Folder size={15} aria-hidden="true" />
   </button>
 
   <div class="repo-tabs-scroll">
@@ -47,7 +46,7 @@
           disabled={isBusy}
           title={repo.path}
         >
-          <Database size={15} aria-hidden="true" />
+          <GitBranch size={13} aria-hidden="true" />
           <span>{repo.name}</span>
         </button>
         <button