Booking flow: required terms and privacy checkbox with i18n

Also includes admin, dashboard, and API updates; PWA icon assets; and
assorted layout and utility changes on dev.

backend/src/lib/utils.ts

@@ -41,6 +41,49 @@ export function toDbDate(date: Date | string): string | Date {
   return getDbType() === 'postgres' ? d : d.toISOString();
 }
 
+const NAIVE_DATETIME_RE = /^\d{4}-\d{2}-\d{2}[T ]\d{2}:\d{2}(:\d{2}(\.\d+)?)?$/;
+
+/**
+ * Parse a datetime string that represents wall-clock time in a given timezone
+ * and return the corresponding UTC Date.
+ *
+ * Naive strings (no "Z" or offset) are interpreted as wall-clock time in
+ * `timezone`.  Strings that already carry a timezone indicator are parsed
+ * directly so existing UTC ISO values still work.
+ */
+export function parseEventDatetime(
+  datetime: string,
+  timezone: string = 'America/Asuncion',
+): Date {
+  if (!NAIVE_DATETIME_RE.test(datetime)) {
+    return new Date(datetime);
+  }
+
+  // Treat the digits as UTC so we have a stable reference instant.
+  const fakeUTC = new Date(datetime + 'Z');
+
+  // Ask Intl what that UTC instant looks like in both UTC and the target tz.
+  const utcStr = fakeUTC.toLocaleString('en-US', { timeZone: 'UTC' });
+  const tzStr  = fakeUTC.toLocaleString('en-US', { timeZone: timezone });
+
+  // The gap between the two tells us the tz offset at this point in time.
+  const offsetMs = new Date(utcStr).getTime() - new Date(tzStr).getTime();
+
+  return new Date(fakeUTC.getTime() + offsetMs);
+}
+
+/**
+ * Convert a datetime string to the appropriate DB format, interpreting naive
+ * strings as wall-clock time in `timezone` (defaults to America/Asuncion).
+ */
+export function toDbDateTz(
+  datetime: string,
+  timezone: string = 'America/Asuncion',
+): string | Date {
+  const d = parseEventDatetime(datetime, timezone);
+  return getDbType() === 'postgres' ? d : d.toISOString();
+}
+
 /**
  * Convert a boolean value to the appropriate format for the database type.
  * - SQLite: returns boolean (true/false) for mode: 'boolean'