feat(scheduling): dezky Scheduling — Calendly-style booking on Stalwart calendars

First-party booking system on top of Stalwart calendars (no third-party
scheduling dependency). Hosts expose public booking pages; visitors pick a
slot computed from the host's live Stalwart free/busy, and confirming writes
the event to the host's calendar and sends a dezky-branded confirmation with
an .ics.

platform-api (services/platform-api/src/scheduling):
- Schemas: Host, StalwartCredential (AES-256-GCM at rest), AvailabilitySchedule,
  EventType, Booking, SlotLock (unique (hostId,startUtc) + TTL).
- StalwartCalendarModule: JMAP gateway (free/busy via Principal/getAvailability,
  event create/delete, scheduleAgent=client) + on-behalf app-password
  provisioning. CredentialCipher for at-rest encryption.
- DST-correct slot engine (Luxon) with unit tests; two-layer double-booking
  guard (atomic SlotLock + live free/busy re-check).
- Booking confirm/cancel/reschedule, branded email + .ics via JMAP submission,
  self-service manage tokens. /api/v1 public + tenant-gated admin routes,
  per-IP rate limiting.

apps/booking: standalone public, whitelabel booking app (booking.dezky.eu) —
path-based tenant resolution, per-tenant brand colour, booking + manage flows.

apps/portal: admin scheduling page (hosts, event types, availability, bookings
with edit/delete + admin cancel/reschedule) and proxy routes.

infra: booking dev service in docker-compose; scheduling env vars.

apps/booking/utils/time.ts

@@ -0,0 +1,67 @@
+import { DateTime } from 'luxon'
+
+// Visitor timezone detection (browser); falls back to UTC on the server / when
+// unavailable. All slot times are transmitted as UTC and rendered in this zone.
+export function detectTimezone(): string {
+  try {
+    return Intl.DateTimeFormat().resolvedOptions().timeZone || 'UTC'
+  } catch {
+    return 'UTC'
+  }
+}
+
+// UTC [from, to) covering one local calendar day in `tz`.
+export function dayWindowUtc(isoDate: string, tz: string): { from: string; to: string } {
+  const start = DateTime.fromISO(isoDate, { zone: tz }).startOf('day')
+  return { from: start.toUTC().toISO()!, to: start.plus({ days: 1 }).toUTC().toISO()! }
+}
+
+export interface DayOption {
+  iso: string // yyyy-MM-dd in tz
+  weekday: string
+  day: string
+  month: string
+}
+
+// The next `count` calendar days starting today, in the visitor's tz.
+export function nextDays(tz: string, count: number): DayOption[] {
+  const today = DateTime.now().setZone(tz).startOf('day')
+  return Array.from({ length: count }, (_, i) => {
+    const d = today.plus({ days: i })
+    return { iso: d.toFormat('yyyy-MM-dd'), weekday: d.toFormat('ccc'), day: d.toFormat('d'), month: d.toFormat('LLL') }
+  })
+}
+
+export function fmtTime(utc: string, tz: string): string {
+  return DateTime.fromISO(utc).setZone(tz).toFormat('HH:mm')
+}
+
+export function fmtDateLong(utc: string, tz: string): string {
+  return DateTime.fromISO(utc).setZone(tz).toFormat('cccc d LLLL yyyy')
+}
+
+export function fmtRange(startUtc: string, endUtc: string, tz: string): string {
+  const s = DateTime.fromISO(startUtc).setZone(tz)
+  const e = DateTime.fromISO(endUtc).setZone(tz)
+  return `${s.toFormat('cccc d LLLL')} · ${s.toFormat('HH:mm')}–${e.toFormat('HH:mm')} (${tz})`
+}
+
+// Google Calendar "add to calendar" template link (UTC basic format).
+export function googleCalUrl(p: { title: string; startUtc: string; endUtc: string; details?: string; location?: string }): string {
+  const fmt = (iso: string) => DateTime.fromISO(iso).toUTC().toFormat("yyyyLLdd'T'HHmmss'Z'")
+  const params = new URLSearchParams({
+    action: 'TEMPLATE',
+    text: p.title,
+    dates: `${fmt(p.startUtc)}/${fmt(p.endUtc)}`,
+  })
+  if (p.details) params.set('details', p.details)
+  if (p.location) params.set('location', p.location)
+  return `https://calendar.google.com/calendar/render?${params.toString()}`
+}
+
+// Common IANA zones for the manual switcher (visitor tz is added if missing).
+export const COMMON_TIMEZONES = [
+  'Europe/Copenhagen', 'Europe/London', 'Europe/Berlin', 'Europe/Paris', 'Europe/Madrid',
+  'Europe/Stockholm', 'Europe/Helsinki', 'America/New_York', 'America/Chicago', 'America/Los_Angeles',
+  'Asia/Dubai', 'Asia/Kolkata', 'Asia/Singapore', 'Asia/Tokyo', 'Australia/Sydney', 'UTC',
+]