feat(integrations): add Git hosting integrations and repo listing

Add support for integrating with external Git hosts (GitLab, Gitea,
and Azure DevOps). The backend gains a client to fetch paginated
repository lists, normalise base URLs, and surface provider errors.
Credentials are loaded from the OS keychain and a Tauri command is
exposed for the frontend to list integration repositories.

- Implement integration client with pagination, deserialization,
  and provider-specific handling.
- Centralise keychain credential loading and expose listing command.
- Update UI to manage integration metadata, persist settings, and
  save/remove tokens to the OS keychain for cloning and operations.

src/lib/git.ts

@@ -15,6 +15,8 @@ import type {
   GitCommit,
   GitCommitComparison,
   GitIgnoreKind,
+  GitIntegrationProvider,
+  GitIntegrationRepository,
   GitLfsStatus,
   GitRepositoryFile,
   GitRemote,
@@ -54,6 +56,10 @@ export function detectExternalTools(): Promise<DetectedExternalTool[]> {
   return invoke<DetectedExternalTool[]>("detect_external_tools");
 }
 
+export function listIntegrationRepositories(provider: GitIntegrationProvider, baseUrl: string, accountId?: string): Promise<GitIntegrationRepository[]> {
+  return invoke<GitIntegrationRepository[]>("list_integration_repositories", { provider, baseUrl, accountId: accountId ?? null });
+}
+
 export function launchExternalTool(path: string, command: ExternalToolCommand, file?: string): Promise<void> {
   return invoke<void>("launch_external_tool", { path, file: file ?? null, command });
 }