feat(07.1-04): add useUserTimezone client hook

- New lib/hooks/use-user-timezone.ts exporting useUserTimezone() and formatInUserTimezone()
- Reads user.timezone from Better Auth useSession() additionalField (Plan 01)
- Validates against Intl.supportedValuesOf('timeZone') with safe fallback to NEXT_PUBLIC_DEFAULT_TIMEZONE || 'UTC'
- Pure formatInUserTimezone helper safe to call inside loops (not a hook)
- Resolves TZ-04

lib/hooks/use-user-timezone.ts

@@ -0,0 +1,61 @@
+"use client";
+
+import { useSession } from "@/lib/auth-client";
+
+// Public-readable default (Next.js exposes NEXT_PUBLIC_* to the browser).
+// Operators can set this in .env.local to match the server-side
+// DEFAULT_TIMEZONE. If unset, both server and client default to 'UTC'.
+function getClientDefaultTimezone(): string {
+  return process.env.NEXT_PUBLIC_DEFAULT_TIMEZONE || "UTC";
+}
+
+function isValidIanaTimezone(tz: unknown): tz is string {
+  if (typeof tz !== "string" || tz.length === 0 || tz.length > 64) return false;
+  try {
+    return Intl.supportedValuesOf("timeZone").includes(tz);
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * useUserTimezone — TZ-04.
+ *
+ * Returns the calling user's IANA timezone string, sourced from the
+ * Better Auth additionalField on `useSession()`. Falls back to
+ * `process.env.NEXT_PUBLIC_DEFAULT_TIMEZONE || 'UTC'` while the session
+ * is loading, when the field is missing, or when the stored value is
+ * not a recognized IANA zone.
+ *
+ * This is the ONLY supported way to read the user's tz on the client.
+ * Do NOT call `Intl.DateTimeFormat()` with a hardcoded zone or rely on
+ * the browser's local zone — the user may have travelled or set a
+ * preference that differs from the device.
+ */
+export function useUserTimezone(): string {
+  const { data } = useSession();
+  // Better Auth additionalField is typed `string` post-Plan-01; defence
+  // in depth: validate before returning.
+  const raw = (data?.user as { timezone?: unknown } | undefined)?.timezone;
+  if (isValidIanaTimezone(raw)) return raw;
+  return getClientDefaultTimezone();
+}
+
+/**
+ * formatInUserTimezone — convenience wrapper for the common case of
+ * "format an ISO string in the user's tz". Equivalent to
+ * `new Date(iso).toLocaleString(locale, { ...options, timeZone: tz })`.
+ *
+ * Pass `tz` from `useUserTimezone()` and the same options object you'd
+ * pass to toLocaleString / toLocaleDateString — this helper keeps
+ * existing format strings working without rewrites.
+ */
+export function formatInUserTimezone(
+  input: string | number | Date,
+  tz: string,
+  options?: Intl.DateTimeFormatOptions,
+  locale: string = "en-US",
+): string {
+  const date = input instanceof Date ? input : new Date(input);
+  return date.toLocaleString(locale, { ...options, timeZone: tz });
+}