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/formula/mod.rs

@@ -1,5 +1,5 @@
 pub mod parser;
 pub mod ast;
 
-pub use ast::{AggFunc, Expr, Filter, Formula};
+pub use ast::{AggFunc, Expr, Formula};
 pub use parser::parse_formula;