Web: expose month-navigation mode as a named dropdown

New 'Monatswechsel' setting (synced flag already existed as month_view_paged):
- Continuous scroll (week): current behaviour — wheel scrolls by a week,
  buttons/swipe jump 4 weeks.
- Page by page (month): wheel navigation is disabled; buttons and horizontal
  swipe jump a whole calendar month.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/js/i18n.js

@@ -92,6 +92,9 @@ const translations = {
     settings_week_start: 'Erster Wochentag',
     week_start_monday: 'Montag', week_start_sunday: 'Sonntag',
     settings_dim_past: 'Vergangene Termine ausgrauen',
+    settings_month_mode: 'Monatswechsel',
+    settings_month_mode_scroll: 'Fortlaufend scrollen (Woche)',
+    settings_month_mode_paged: 'Seitenweise blättern (Monat)',
     settings_default_duration: 'Standard-Termindauer',
     settings_privacy: 'Privatsphäre',
     settings_private_visibility: 'Private Termine für Gruppenmitglieder',
@@ -446,6 +449,9 @@ const translations = {
     settings_week_start: 'First day of week',
     week_start_monday: 'Monday', week_start_sunday: 'Sunday',
     settings_dim_past: 'Dim past events',
+    settings_month_mode: 'Month switching',
+    settings_month_mode_scroll: 'Continuous scroll (week)',
+    settings_month_mode_paged: 'Page by page (month)',
     settings_default_duration: 'Default event duration',
     settings_privacy: 'Privacy',
     settings_private_visibility: 'Private events for group members',