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

diff --git a/src/app.rs b/src/app.rs
index cb62682..f87a189 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -903,7 +903,7 @@ pub struct State {
     pub last_saved_root_json: String,
     pub recent_files: Vec<std::path::PathBuf>,
     pub recent_files_list: List,
-    pub recent_files_buttons: Vec<Button>,
+    pub recent_files_buttons: Vec<cce_ui::widget::Adapted<cce_ui::widget::Button>>,
     pub text_buffer_cache: std::collections::HashMap<(String, u32, Option<String>), Buffer>,
     pub viewport_dirty: bool,
     pub text_dirty: bool,