Chore: remove unused imports and suppress unused variable warnings

Removes dead use statements across dispatch, formula, import, model, and
UI modules. Prefixes intentionally unused variables with _ in app.rs,
analyzer.rs, and grid.rs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/command/dispatch.rs

@@ -1,4 +1,3 @@
-use anyhow::Result;
 
 use crate::model::Model;
 use crate::model::cell::{CellKey, CellValue};