feat(webui): add initial webui with websocket chat flow

webui/src/lib/format.ts

@@ -0,0 +1,44 @@
+/** Truncate the first user message into a chat title. */
+export function deriveTitle(preview: string | undefined, fallback: string): string {
+  if (!preview) return fallback;
+  const oneLine = preview.replace(/\s+/g, " ").trim();
+  if (!oneLine) return fallback;
+  return oneLine.length > 60 ? `${oneLine.slice(0, 57)}…` : oneLine;
+}
+
+/** Loose ISO-or-epoch parser; returns ``null`` for missing/invalid input. */
+function parseDate(value: string | number | null | undefined): Date | null {
+  if (value === null || value === undefined || value === "") return null;
+  const d = new Date(value);
+  return Number.isNaN(d.getTime()) ? null : d;
+}
+
+const RELATIVE_THRESHOLDS: [number, Intl.RelativeTimeFormatUnit][] = [
+  [60, "second"],
+  [60, "minute"],
+  [24, "hour"],
+  [7, "day"],
+  [4.345, "week"],
+  [12, "month"],
+  [Number.POSITIVE_INFINITY, "year"],
+];
+
+const RTF = new Intl.RelativeTimeFormat(undefined, { numeric: "auto" });
+
+export function relativeTime(value: string | number | null | undefined): string {
+  const date = parseDate(value);
+  if (!date) return "";
+  let delta = (date.getTime() - Date.now()) / 1000;
+  for (const [step, unit] of RELATIVE_THRESHOLDS) {
+    if (Math.abs(delta) < step) {
+      return RTF.format(Math.round(delta), unit);
+    }
+    delta /= step;
+  }
+  return RTF.format(Math.round(delta), "year");
+}
+
+export function fmtDateTime(value: string | number | null | undefined): string {
+  const date = parseDate(value);
+  return date ? date.toLocaleString() : "";
+}