refactor: extract GridLayout as single source for view→grid mapping

Three copies of cross_product existed (grid.rs, app.rs, persistence/mod.rs)
with slightly different signatures. Extracted into GridLayout in
src/view/layout.rs, which is now the single canonical mapping from a View
to a 2-D grid: row/col counts, labels, and cell_key(row, col) → CellKey.

All consumers updated to use GridLayout::new(model, view):
- grid.rs: render_grid, total-row computation, page bar
- persistence/mod.rs: export_csv
- app.rs: move_selection, jump_to_last_row/col, scroll_rows,
  search_navigate, selected_cell_key

Also includes two app.rs UI bug fixes that were discovered while
refactoring:
- Ctrl+Arrow tile movement was unreachable (shadowed by plain arrow arms);
  moved before plain arrow handlers
- RemoveFormula dispatch now passes target_category (required by the
  formula management fix in the previous commit)

GridLayout has 6 unit tests covering counts, label formatting, cell_key
correctness, out-of-bounds, page coord inclusion, and evaluate round-trip.

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

src/view/mod.rs

@@ -1,5 +1,7 @@
 pub mod view;
 pub mod axis;
+pub mod layout;
 
 pub use view::View;
 pub use axis::Axis;
+pub use layout::GridLayout;