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/view/layout.rs

@@ -228,14 +228,10 @@ impl GridLayout {
         let mut has_value = vec![vec![false; cc]; rc];
         for ri in 0..rc {
             for ci in 0..cc {
-                has_value[ri][ci] = if self.is_records_mode() {
-                    let s = self.records_display(ri, ci).unwrap_or_default();
-                    !s.is_empty()
-                } else {
-                    self.cell_key(ri, ci)
-                        .and_then(|k| model.evaluate_aggregated(&k, &self.none_cats))
-                        .is_some()
-                };
+                has_value[ri][ci] = self
+                    .cell_key(ri, ci)
+                    .and_then(|k| model.evaluate_aggregated(&k, &self.none_cats))
+                    .is_some();
             }
         }
 
@@ -373,6 +369,27 @@ impl GridLayout {
         }
     }
 
+    /// Unified display text for a cell at (row, col). Handles both pivot and
+    /// records modes. In pivot mode, evaluates and formats the cell value.
+    /// In records mode, resolves via the frozen records snapshot.
+    pub fn display_text(
+        &self,
+        model: &Model,
+        row: usize,
+        col: usize,
+        fmt_comma: bool,
+        fmt_decimals: u8,
+    ) -> String {
+        if self.is_records_mode() {
+            self.records_display(row, col).unwrap_or_default()
+        } else {
+            self.cell_key(row, col)
+                .and_then(|key| model.evaluate_aggregated(&key, &self.none_cats))
+                .map(|v| crate::format::format_value(Some(&v), fmt_comma, fmt_decimals))
+                .unwrap_or_default()
+        }
+    }
+
     /// Build the CellKey for the data cell at (row, col), including the active
     /// page-axis filter. Returns None if row or col is out of bounds.
     /// In records mode: returns a synthetic `(_Index, _Dim)` key for every column.