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/integrations.ts

@@ -7,6 +7,7 @@ import type {
 } from "./types";
 
 export const gitIntegrationProviders: GitIntegrationProvider[] = [
+  "github",
   "gitlab",
   "gitlab-self-hosted",
   "azure-devops",
@@ -14,6 +15,12 @@ export const gitIntegrationProviders: GitIntegrationProvider[] = [
 ];
 
 const defaults: Record<GitIntegrationProvider, Omit<GitIntegrationConfig, "provider">> = {
+  github: {
+    enabled: false,
+    baseUrl: "https://github.com",
+    username: "",
+    tokenStored: false,
+  },
   gitlab: {
     enabled: false,
     baseUrl: "https://gitlab.com",
@@ -144,6 +151,7 @@ export function configuredIntegrationCount(settings: GitIntegrationSettings): nu
 
 export function providerLabel(provider: GitIntegrationProvider): string {
   return {
+    github: "GitHub",
     gitlab: "GitLab.com",
     "gitlab-self-hosted": "GitLab Self-Managed",
     "azure-devops": "Azure DevOps",