feat: number formatting, real search navigation, CSV cross-product fix

- grid.rs: honour view.number_format (",.0" default, ",.2", ".4", etc.)
  via parse_number_format/format_f64(n,comma,decimals); format_f64 now
  pub so callers can reuse the same formatting logic.
- app.rs: n/N actually navigate to next/prev search match (cross-product
  aware); fix dead unreachable N arm; add :set-format / :fmt command to
  change the active view's number_format at runtime.
- persistence/mod.rs: CSV export now uses full cross-product of all
  row/col-axis categories, matching grid rendering behaviour.

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

src/ui/grid.rs

@@ -61,6 +61,7 @@ impl<'a> GridWidget<'a> {
         let (sel_row, sel_col) = view.selected;
         let row_offset = view.row_offset;
         let col_offset = view.col_offset;
+        let (fmt_comma, fmt_decimals) = parse_number_format(&view.number_format);
 
         // Available cols
         let available_cols = ((area.width.saturating_sub(ROW_HEADER_WIDTH)) / COL_WIDTH) as usize;
@@ -133,7 +134,7 @@ impl<'a> GridWidget<'a> {
                 let key = CellKey::new(coords);
                 let value = self.model.evaluate(&key);
 
-                let cell_str = format_value(&value);
+                let cell_str = format_value(&value, fmt_comma, fmt_decimals);
                 let is_selected = abs_ri == sel_row && abs_ci == sel_col;
                 let is_search_match = !self.search_query.is_empty()
                     && cell_str.to_lowercase().contains(&self.search_query.to_lowercase());
@@ -195,7 +196,7 @@ impl<'a> GridWidget<'a> {
                         self.model.evaluate(&key).as_f64().unwrap_or(0.0)
                     }).sum();
 
-                    let total_str = format_f64(total);
+                    let total_str = format_f64(total, fmt_comma, fmt_decimals);
                     buf.set_string(x, y,
                         format!("{:>width$}", truncate(&total_str, COL_WIDTH as usize), width = COL_WIDTH as usize),
                         Style::default().fg(Color::Yellow).add_modifier(Modifier::BOLD));
@@ -281,34 +282,46 @@ impl<'a> Widget for GridWidget<'a> {
     }
 }
 
-fn format_value(v: &CellValue) -> String {
+fn format_value(v: &CellValue, comma: bool, decimals: u8) -> String {
     match v {
-        CellValue::Number(n) => format_f64(*n),
+        CellValue::Number(n) => format_f64(*n, comma, decimals),
         CellValue::Text(s) => s.clone(),
         CellValue::Empty => String::new(),
     }
 }
 
-fn format_f64(n: f64) -> String {
-    if n == 0.0 {
-        return "0".to_string();
+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)
+}
+
+pub fn format_f64(n: f64, comma: bool, decimals: u8) -> String {
+    let formatted = format!("{:.prec$}", n, prec = decimals as usize);
+    if !comma {
+        return formatted;
     }
-    if n.fract() == 0.0 && n.abs() < 1e12 {
-        // Integer with comma formatting
-        let i = n as i64;
-        let s = i.to_string();
-        let is_neg = s.starts_with('-');
-        let digits = if is_neg { &s[1..] } else { &s[..] };
-        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('-'); }
-        result.chars().rev().collect()
+    // Split integer and decimal parts
+    let (int_part, dec_part) = if let Some(dot) = formatted.find('.') {
+        (&formatted[..dot], Some(&formatted[dot..]))
     } else {
-        format!("{n:.2}")
+        (&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
 }
 
 fn truncate(s: &str, max_width: usize) -> String {