chore: drop dead scroll_mut helper (ScrollController removed from cce-ui)

ScrollController had zero implementors workspace-wide; this accessor would
have panicked on any call and had no callers.

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

 src/app.rs | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/src/app.rs b/src/app.rs
index f87a189..e640955 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -1033,10 +1033,6 @@ impl State {
         self.widgets[idx].as_geom_controller_mut().expect("not a GeomController")
     }
 
-    pub fn scroll_mut(&mut self, idx: usize) -> &mut dyn cce_ui::widget::ScrollController {
-        self.widgets[idx].as_scroll_controller_mut().expect("not a ScrollController")
-    }
-
     pub fn has_unsaved_changes(&self) -> bool {
         if let Ok(current_json) = serde_json::to_string(&self.fs_root) {
             current_json != self.last_saved_root_json