Initial commit: scan_web integrated recording UI

FastAPI backend + vanilla JS frontend for LiDAR/camera startup, camera
settings, and rosbag recording, unifying the previous scan_gui/scan_gui_dual/
scan_gui_triple desktop tools into one web app.

frontend/js/format.js

@@ -0,0 +1,21 @@
+export function formatBytes(bytes) {
+  if (bytes == null) return "";
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(0)} KB`;
+  if (bytes < 1024 * 1024 * 1024) return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+  return `${(bytes / (1024 * 1024 * 1024)).toFixed(2)} GB`;
+}
+
+export function formatElapsed(seconds) {
+  if (seconds == null) return "";
+  const s = Math.floor(seconds);
+  const m = Math.floor(s / 60);
+  const h = Math.floor(m / 60);
+  const pad = (n) => String(n).padStart(2, "0");
+  return h > 0 ? `${h}:${pad(m % 60)}:${pad(s % 60)}` : `${pad(m)}:${pad(s % 60)}`;
+}
+
+export function formatDuration(seconds) {
+  if (seconds == null) return "—";
+  if (seconds < 60) return `${seconds.toFixed(1)}s`;
+  return formatElapsed(seconds);
+}