refactor: use chrono for month names

src/timesheet/report.rs

@@ -154,22 +154,10 @@ impl MonthReport {
     }
 
     /// Get the month name.
-    pub fn month_name(&self) -> &'static str {
-        match self.month {
-            1 => "January",
-            2 => "February",
-            3 => "March",
-            4 => "April",
-            5 => "May",
-            6 => "June",
-            7 => "July",
-            8 => "August",
-            9 => "September",
-            10 => "October",
-            11 => "November",
-            12 => "December",
-            _ => "Unknown",
-        }
+    pub fn month_name(&self) -> String {
+        NaiveDate::from_ymd_opt(self.year, self.month, 1)
+            .map(|d| d.format("%B").to_string())
+            .unwrap_or_else(|| "Unknown".to_string())
     }
 }