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/model/mod.rs

@@ -2,6 +2,5 @@ pub mod category;
 pub mod cell;
 pub mod model;
 
-pub use category::{Category, CategoryId, Group, Item, ItemId};
-pub use cell::{CellKey, CellValue, DataStore};
+pub use cell::CellKey;
 pub use model::Model;

src/model/model.rs

@@ -1,4 +1,3 @@
-use std::collections::HashMap;
 use indexmap::IndexMap;
 use serde::{Deserialize, Serialize};
 use anyhow::{anyhow, Result};