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/format.rs

@@ -0,0 +1,50 @@
+use crate::model::cell::CellValue;
+
+/// Format a CellValue for display with number formatting options.
+pub fn format_value(v: Option<&CellValue>, comma: bool, decimals: u8) -> String {
+    match v {
+        Some(CellValue::Number(n)) => format_f64(*n, comma, decimals),
+        Some(CellValue::Text(s)) => s.clone(),
+        None => String::new(),
+    }
+}
+
+/// Parse a number format string like ",.0" into (use_commas, decimal_places).
+pub fn parse_number_format(fmt: &str) -> (bool, u8) {
+    let comma = fmt.contains(',');
+    let decimals = fmt
+        .rfind('.')
+        .and_then(|i| fmt[i + 1..].parse::<u8>().ok())
+        .unwrap_or(0);
+    (comma, decimals)
+}
+
+/// Format an f64 with optional comma grouping and decimal places.
+pub fn format_f64(n: f64, comma: bool, decimals: u8) -> String {
+    let formatted = format!("{:.prec$}", n, prec = decimals as usize);
+    if !comma {
+        return formatted;
+    }
+    let (int_part, dec_part) = if let Some(dot) = formatted.find('.') {
+        (&formatted[..dot], Some(&formatted[dot..]))
+    } else {
+        (&formatted[..], None)
+    };
+    let is_neg = int_part.starts_with('-');
+    let digits = if is_neg { &int_part[1..] } else { int_part };
+    let mut result = String::new();
+    for (idx, c) in digits.chars().rev().enumerate() {
+        if idx > 0 && idx % 3 == 0 {
+            result.push(',');
+        }
+        result.push(c);
+    }
+    if is_neg {
+        result.push('-');
+    }
+    let mut out: String = result.chars().rev().collect();
+    if let Some(dec) = dec_part {
+        out.push_str(dec);
+    }
+    out
+}