refactor: adapt Button fields to the cce-ui Phase 5 migration

Button::new now returns Adapted<Button>; concrete-typed fields become
Adapted<Button>. Construction sites, builder chains, and method calls
are unchanged (Deref + Element forwards + mirrored builders).

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01LkfJazPs9bRchkcozmxXCX

 src/main.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/main.rs b/src/main.rs
index d9cd20a..9286a81 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -336,7 +336,7 @@ struct State {
     card: LoginCard,
     username_box: TextBox,
     password_box: TextBox,
-    login_btn: Button,
+    login_btn: cce_ui::widget::Adapted<cce_ui::widget::Button>,
     status_lbl: StatusLabel,
     session_list: SessionList,
     ui_context: cce_ui::context::UiContext,