refactor(view): unify cell handling for records and pivot modes

Unifies cell text retrieval and formatting across pivot and records modes.
Introduces `GridLayout::display_text` to centralize how cell content is
resolved, reducing duplication in `GridWidget` and `export_csv`.
Moves formatting logic from `src/ui/grid.rs` to a new dedicated `src/format.rs`
module to improve reusability.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-31B-it-GGUF:UD-Q5_K_XL)

src/main.rs

@@ -1,5 +1,6 @@
 mod command;
 mod draw;
+mod format;
 mod formula;
 mod import;
 mod model;