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]/newsletter/templates/page.tsx

@@ -2,6 +2,7 @@ import { FileText, Plus } from 'lucide-react';
 import { getTranslations } from 'next-intl/server';
 
 import { createNewsletterApi } from '@kit/newsletter/api';
+import { CreateTemplateDialog } from '@kit/newsletter/components';
 import { getSupabaseServerClient } from '@kit/supabase/server-client';
 import { Badge } from '@kit/ui/badge';
 import { Button } from '@kit/ui/button';
@@ -41,10 +42,7 @@ export default async function NewsletterTemplatesPage({ params }: PageProps) {
             <p className="text-muted-foreground">{t('templates.subtitle')}</p>
           </div>
 
-          <Button data-test="newsletter-templates-new-btn">
-            <Plus className="mr-2 h-4 w-4" />
-            {t('templates.newTemplate')}
-          </Button>
+          <CreateTemplateDialog accountId={acct.id} />
         </div>
 
         {/* Table or Empty State */}