Deduplicate time formatting and clean up unused code

Extract shared TimeInterval.formattedAsTimestamp replacing 8 identical
formatTime/formattedTime implementations across player views. Remove
unused currentTime parameter from GestureSeekPreviewView. Consolidate
duplicated geometry math in MacOSControlBar into seekPreviewPosition().

Yattee/Extensions/TimeInterval+Formatting.swift

@@ -0,0 +1,16 @@
+import Foundation
+
+extension TimeInterval {
+    /// Formats as "M:SS" or "H:MM:SS" when hours > 0.
+    var formattedAsTimestamp: String {
+        let totalSeconds = Int(max(0, self))
+        let hours = totalSeconds / 3600
+        let minutes = (totalSeconds % 3600) / 60
+        let seconds = totalSeconds % 60
+
+        if hours > 0 {
+            return String(format: "%d:%02d:%02d", hours, minutes, seconds)
+        }
+        return String(format: "%d:%02d", minutes, seconds)
+    }
+}