feat(config): add configurable date format

- Add dateFormat property to Config with ISO 8601 default
- Use config.dateFormat for both sample text and live clock
- Improve sample date generation to test edge case (UTC-12 timezone)

Sources/Nunc/main.swift

@@ -12,6 +12,7 @@ struct Config: Codable {
     var position: Position = .top_right
     var offsetX: Double = 0.0
     var offsetY: Double = 0.0
+    var dateFormat: String = "yyyy-MM-dd'T'HH:mm:ssXXXXX"
 }
 
 func loadConfig() -> Config {
@@ -65,7 +66,21 @@ label.alignment = .center
 label.translatesAutoresizingMaskIntoConstraints = false
 
 // Sample Dimensions
-let sampleText = "2026-04-15T23:59:59+00:00"
+let sampleFormatter = DateFormatter()
+sampleFormatter.dateFormat = config.dateFormat
+sampleFormatter.locale = Locale(identifier: "en_US_POSIX")
+let sampleDate = {
+    var components = DateComponents()
+    components.year = 2026
+    components.month = 12
+    components.day = 30
+    components.hour = 23
+    components.minute = 59
+    components.second = 59
+    components.timeZone = TimeZone(secondsFromGMT: -12 * 3600) // Widest offset
+    return Calendar.current.date(from: components) ?? Date()
+}()
+let sampleText = sampleFormatter.string(from: sampleDate)
 let textSize = (sampleText as NSString).size(withAttributes: [.font: font])
 let windowSize = NSSize(
     width: ceil(textSize.width + config.horizontalPadding * 2),
@@ -135,7 +150,7 @@ window.contentView = vfx
 window.orderFrontRegardless()
 
 let formatter = DateFormatter()
-formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssXXXXX"
+formatter.dateFormat = config.dateFormat
 formatter.locale = Locale(identifier: "en_US_POSIX")
 
 func updateClock() {