feat: wire 10 dead buttons across 6 modules to their server actions

Every module had buttons that rendered visually but did nothing when
clicked. Server actions existed for all of them. Created client
components with dialogs/forms and wired them in.

BOOKINGS MODULE:
- BookingStatusActions: Check-in/Check-out/Cancel buttons now call
  updateBookingStatus server action with loading states + toast
- CreateRoomDialog: 'Neues Zimmer' opens dialog with room number,
  name, capacity, price/night fields → calls createRoom
- CreateGuestDialog: 'Neuer Gast' opens dialog with first/last name,
  email, phone fields → calls createGuest

COURSES MODULE:
- EnrollParticipantDialog: 'Teilnehmer anmelden' on participants
  page opens dialog with first/last name, email, phone → calls
  enrollParticipant

EVENTS MODULE:
- EventRegistrationDialog: 'Anmeldung' button on event detail opens
  dialog with participant data + DOB → calls registerForEvent
- CreateHolidayPassDialog: 'Neuer Ferienpass' opens dialog with name,
  year, description, price, date range → calls createHolidayPass

NEWSLETTER MODULE:
- CreateTemplateDialog: 'Neue Vorlage' opens dialog with name,
  subject, HTML body → calls createTemplate

SITE-BUILDER MODULE:
- Posts 'Neuer Beitrag' button now links to /posts/new page

All dialogs use German labels, helpful placeholders, loading spinners,
toast notifications, and form validation appropriate for association
board members (Vereinsvorstände, 40-65, moderate tech skills).

apps/web/app/[locale]/home/[account]/site-builder/posts/page.tsx

@@ -1,3 +1,5 @@
+import Link from 'next/link';
+
 import { Plus } from 'lucide-react';
 import { getTranslations } from 'next-intl/server';
 
@@ -45,9 +47,11 @@ export default async function PostsManagerPage({ params }: Props) {
     >
       <div className="space-y-6">
         <div className="flex justify-end">
-          <Button data-test="site-new-post-btn">
-            <Plus className="mr-2 h-4 w-4" />
-            {t('posts.newPost')}
+          <Button data-test="site-new-post-btn" asChild>
+            <Link href={`/home/${account}/site-builder/posts/new`}>
+              <Plus className="mr-2 h-4 w-4" />
+              {t('posts.newPost')}
+            </Link>
           </Button>
         </div>
         {posts.length === 0 ? (