fix(tui): route each OAuth picker row to its own provider, not kimi.com

Root cause: PendingMenuItem::Login carried only a label, so every provider
row collapsed to the id-less Action::Login, and dispatch_login resolved
the FIRST advertised interactive method — the Kimi device flow. Selecting
Grok/Claude/Copilot/Codex all opened kimi.com. (The shell side was already
correct: authenticate() dispatches each method id to its own OAuth flow.)

- PendingMenuItem::Login now carries the advertised method id (None only
  on the no-interactive-method fallback row).
- New Action::LoginWith(AuthMethodId); the picker dispatches it with the
  row's own id. Action::Login keeps its meaning (resolved/default method)
  for /login, auto-login, and re-auth.
- dispatch_login_with resolves the id against the advertised methods and
  FAILS CLOSED on an unknown id — no silent first-method fallback — then
  adopts the method's label and start mode.

Tests: picker rows pinned to their method ids; LoginWith(claude-pro-max)
must authenticate with claude-pro-max even when kimi-code was previously
resolved; unknown ids surface an error and start nothing.

Verified: kigi-tui 6867 tests green, clippy clean.

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

@@ -562,6 +562,11 @@ pub enum Action {
     SwitchAccount,
     /// User pressed login on the welcome screen.
     Login,
+    /// User chose a specific OAuth provider row on the login picker. Routes
+    /// the flow to THAT advertised method — `Login` alone resolves to the
+    /// first interactive method, which sent every provider row to the Kimi
+    /// device flow.
+    LoginWith(acp::AuthMethodId),
     /// Cancel an in-progress login that was started from inside a session
     /// (`/login` or a 401 re-auth prompt) and return to the previous view.
     /// Distinct from `Quit`: abandoning a mid-session re-auth must not exit