Show cell coordinates with column letters

Introduce `col_to_letters` to convert a zero‑based column index into
Excel‑style
letters. Add `display_coords` that returns the column letters followed
by the
row number. Update `render` to use this formatted coordinate as the
block
title instead of the previous row/col placeholder.

src/app/widgets/cell.rs

@@ -24,13 +24,34 @@ impl CellWidget {
     pub fn new(row: usize, col: usize) -> Self {
         Self { row, col }
     }
+
+    fn col_to_letters(&self) -> String {
+        let mut col: usize = self.col + 1;
+        let mut letters: Vec<char> = Vec::new();
+
+        while col > 0 {
+            letters.push((b'A' + ((col - 1) % 26) as u8) as char);
+            col = (col - 1) / 26;
+        }
+
+        letters.iter().rev().collect()
+    }
+
+    pub fn display_coords(&self) -> String {
+        format!("{}{}", self.col_to_letters(), self.row)
+    }
 }
 
 impl Widget for CellWidget {
     fn render(self, area: Rect, buf: &mut Buffer) {
-        Paragraph::new(format!("{}/{}", self.row, self.col))
+        Paragraph::new("")
             .alignment(Alignment::Center)
-            .block(Block::default().borders(Borders::ALL).white())
+            .block(
+                Block::default()
+                    .borders(Borders::ALL)
+                    .white()
+                    .title(self.display_coords().green()),
+            )
             .render(area, buf);
     }
 }