Login picker: offer all three platforms like the official CLI

The unauthenticated welcome screen previously offered only 'Login with
Kimi Code'. It now lists every interactive platform the shell
advertises, matching the official kimi-cli picker:

  Kimi Code (OAuth)
  Moonshot Open Platform (API key · moonshot.cn)
  Moonshot Open Platform (API key · moonshot.ai)
  Quit

- Shell: new ACP auth methods moonshot-cn / moonshot-ai (advertised
  after kimi-code; the BYOK first-position invariant holds).
  authenticate(moonshot-*) reloads keys from env>config, fails with an
  actionable message when none is configured, validates the key against
  GET {base}/models (401 → 'invalid API key' naming the console), then
  swaps the fresh config in, triggers the model sync, and reports
  auth_mode api_key so the pager treats it like other API-key logins.
  Never session-based; keys never logged.
- Config: save_platform_api_key persists [platforms.<id>].api_key via an
  atomic mode-preserving write under the config lock; refuses OAuth
  platforms, blank keys, and unparseable files.
- TUI: the Pending welcome renders the picker rows from the advertised
  methods (arrows/Enter/mouse; 'l' keeps selecting the OAuth row).
  Choosing a Moonshot row opens a masked paste box ('Paste your Moonshot
  API key (from platform.moonshot.cn)'); Esc returns to the picker,
  Enter persists the key and authenticates in one sequential effect;
  failures return to the picker with the error line, success lands on
  the normal welcome. Startup eager-auth is unchanged: a key already in
  the environment authenticates exactly as before, and single-method
  shells keep the historical auto-device-flow.

Gates: workspace check/clippy 0/0; shell 4870 + tui 6620 lib tests
green; headless probe advertises [kimi-code, moonshot-cn, moonshot-ai]
for a fresh user and xai.api_key-first with a key configured.

crates/codegen/kigi-tui/src/app/event_loop.rs

@@ -683,6 +683,13 @@ pub(crate) async fn run(
                 error: Some("No login method available".to_string()),
             };
             vec![]
+        } else if super::app_view::login_picker_has_choice(&app.auth_methods) {
+            // Multiple interactive login choices (OAuth + Moonshot API-key
+            // rows): land on the login picker instead of auto-starting the
+            // device flow, so the user can choose a platform. Single-choice
+            // shells keep the historical auto-trigger below.
+            app.auth_state = super::app_view::AuthState::Pending { error: None };
+            vec![]
         } else {
             dispatch::dispatch(Action::Login, &mut app)
         }