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-tauri/src/main.rs

@@ -3,6 +3,7 @@
 mod badge;
 mod external_tools;
 mod git;
+mod integrations;
 mod telemetry;
 
 use badge::set_sync_badge;
@@ -32,6 +33,7 @@ use git::{
     stash_drop, stash_pop, stash_push, undo_last_commit, unlock_worktree, unstage_files,
     untrack_paths, update_remote,
 };
+use integrations::list_integration_repositories;
 use std::path::{Path, PathBuf};
 use std::sync::Mutex;
 use tauri::{Emitter, Manager};
@@ -432,6 +434,7 @@ async fn main() {
             cred_load,
             cred_save,
             cred_delete,
+            list_integration_repositories,
             set_sync_badge,
             close_splashscreen,
             set_telemetry_enabled,