refactor: remove dead code, replace sum_matching tests with evaluate()

Removes unused methods (sum_matching, get_mut, item_by_name, item_index,
top_level_groups, is_group_collapsed, show_item) and unused constants
(LABEL_THRESHOLD, MIN_COL_WIDTH).

The sum_matching tests in model.rs were bypassing the formula evaluator
entirely. Replaced them with equivalent tests that call evaluate() against
the existing Total = SUM(Revenue) formula, exercising the real aggregation
code path.

Also fixes a compile error in view.rs prop_tests where View/Axis imports
and a doc comment were incorrectly commented out.

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

src/ui/grid.rs

@@ -13,7 +13,6 @@ use crate::ui::app::AppMode;
 
 const ROW_HEADER_WIDTH: u16 = 16;
 const COL_WIDTH: u16 = 10;
-const MIN_COL_WIDTH: u16 = 6;
 
 pub struct GridWidget<'a> {
     pub model: &'a Model,