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/import/analyzer.rs

@@ -75,7 +75,7 @@ pub fn analyze_records(records: &[Value]) -> Vec<FieldProposal> {
                 .collect();
             let distinct_vec: Vec<String> = distinct.into_iter().map(String::from).collect();
             let n = distinct_vec.len();
-            let total = values.len();
+            let _total = values.len();
 
             // Check if looks like date
             let looks_like_date = distinct_vec.iter().any(|s| {