feat(calendar): add feed subscription, bulk export, share & Google Cal

- /api/kalender.ics SSR route: ICS feed of all calendar items (webcal-subscribable, Cache 1h)
- buildICSFeed() / downloadICSFeed() for multi-event ICS export
- googleCalUrl() deep-link pre-fills Google Calendar create form
- Web Share API per event (navigator.share on mobile, clipboard fallback on desktop)
  — "Kopiert\!" toast per event for 2s after clipboard write
- Actions bar (subscribe + bulk export) only rendered after onMount
  — Apple platforms: webcal:// link opens Calendar.app natively
  — All others: direct ICS download via <a download>

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/translations.ts

@@ -137,6 +137,13 @@ const TRANSLATION_KEYS = [
   "calendar_untitled",
   "calendar_show_more",
   "calendar_show_less",
+  "calendar_subscribe",
+  "calendar_subscribe_aria",
+  "calendar_export_all",
+  "calendar_add_to_google",
+  "calendar_share",
+  "calendar_share_copied",
+  "calendar_share_aria",
   "post_overview_all",
   "blog_search_label",
   "blog_search_placeholder",