feat(login): custom provider logins fetch and select models

After the base URL, name, and key steps, the login now opens a model
selection screen instead of blindly saving. The listing fetch runs
concurrently with a manual-id input — a WAF challenge page or a 404 no
longer passes validation silently: a base without a /vN tail is probed
once as {base}/v1 and the working base is adopted, an anthropic-wire
listing that 401s on x-api-key retries once with Bearer (relays gate
GET /models on Bearer while /messages takes the key header), and a
failed fetch shows its reason while manual add stays usable. The chosen
ids persist as [platforms.<name>] models = [...]; catalog assembly
synthesizes selected ids the listing never served (before [model.*]
overrides so they compose) and filters unselected provider entries
(after overrides, authoritative in the provider namespace). Provider-
owned entries keep their declared wire against donor backend
inheritance, and listing errors quote line/column only — a reflected
body could carry the key.

crates/codegen/kigi-tui/src/app/dispatch/task_result.rs

@@ -509,9 +509,14 @@ pub(super) fn dispatch_task_result(result: TaskResult, app: &mut AppView) -> Vec
             {
                 app.auth_state = AuthState::Pending { error: Some(error) };
                 app.auth_code_input.clear();
+                app.custom_select = None;
             }
             vec![]
         }
+        TaskResult::CustomProviderModelsListed {
+            request_seq,
+            result,
+        } => super::auth::handle_custom_provider_models_listed(app, request_seq, result),
         TaskResult::AuthUrlReady {
             request_seq,
             auth_url,
@@ -798,6 +803,7 @@ pub(super) fn dispatch_task_result(result: TaskResult, app: &mut AppView) -> Vec
             app.login_method_id = None;
             ensure_login_method(app);
             app.auth_clipboard_copied = false;
+            app.custom_select = None;
             let effects = dispatch_exit_session(app);
             app.welcome_prompt_focused = false;
             effects