feat(web): add formatDuration helper

Renders seconds as mm:ss or h:mm:ss. Negative inputs clamp to 0:00
so a bad backend value doesn't crash the UI.

web/src/lib/media/duration.ts

@@ -0,0 +1,15 @@
+// Formats a non-negative number of seconds as `mm:ss` when under an hour,
+// `h:mm:ss` otherwise. Negative values clamp to zero (callers shouldn't
+// pass them, but the UI shouldn't crash if they do).
+export function formatDuration(seconds: number): string {
+  const total = Math.max(0, Math.floor(seconds));
+  const h = Math.floor(total / 3600);
+  const m = Math.floor((total % 3600) / 60);
+  const s = total % 60;
+  const ss = String(s).padStart(2, '0');
+  if (h > 0) {
+    const mm = String(m).padStart(2, '0');
+    return `${h}:${mm}:${ss}`;
+  }
+  return `${m}:${ss}`;
+}