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/git.rs

@@ -2878,6 +2878,17 @@ fn cred_entry(key: &str) -> Result<keyring::Entry, String> {
     keyring::Entry::new(CRED_SERVICE, key).map_err(|err| format!("Keychain unavailable: {err}"))
 }
 
+pub(crate) fn load_stored_credential(key: &str) -> Result<Option<StoredCredential>, String> {
+    let entry = cred_entry(key)?;
+    match entry.get_password() {
+        Ok(json) => serde_json::from_str::<StoredCredential>(&json)
+            .map(Some)
+            .map_err(|err| format!("Stored credentials unreadable: {err}")),
+        Err(keyring::Error::NoEntry) => Ok(None),
+        Err(err) => Err(format!("Keychain access failed: {err}")),
+    }
+}
+
 /// Returns the remote URL used for auth key derivation (upstream remote of the
 /// current branch, falling back to `origin`, then the first configured remote).
 #[tauri::command(async)]
@@ -3061,16 +3072,7 @@ fn push_args_for_repo_to(
 
 #[tauri::command(async)]
 pub fn cred_load(key: String) -> Result<Option<StoredCredential>, String> {
-    let entry = cred_entry(&key)?;
-    match entry.get_password() {
-        Ok(json) => {
-            let cred = serde_json::from_str::<StoredCredential>(&json)
-                .map_err(|err| format!("Stored credentials unreadable: {err}"))?;
-            Ok(Some(cred))
-        }
-        Err(keyring::Error::NoEntry) => Ok(None),
-        Err(err) => Err(format!("Keychain access failed: {err}")),
-    }
+    load_stored_credential(&key)
 }
 
 #[tauri::command(async)]