feat: Spanning event bars, wheel nav, dark datetime picker, segmented settings UI

- Month view: Multi-day events render as continuous Google Calendar-style
  spanning bars across days/weeks using a greedy lane-packing algorithm.
  Timed multi-day events no longer repeat per day.
- Mouse wheel / trackpad scrolls week-by-week in month view, day/week in
  other views (debounced, prevents default page scroll).
- datetime-local/date inputs now use color-scheme:dark so the native
  browser picker opens in dark mode; calendar icon styled to match.
- Contrast/hour-height selectors redesigned as connected segmented pill
  controls instead of individual tiles.
- Hidden calendars list gains proper padding and separator lines.
- "Google Konten" settings panel renamed "Konten" and expanded to show
  CalDAV, local calendars, iCal subscriptions, and Google accounts in
  one unified panel with sync/disconnect actions.
- New i18n keys added for accounts panel in both de and en.

frontend/js/i18n.js

@@ -86,6 +86,17 @@ const translations = {
     settings_hidden_cals: 'Ausgeblendete Kalender',
     settings_no_hidden_cals: 'Keine ausgeblendeten Kalender',
     settings_no_google: 'Keine Google-Konten verbunden',
+    settings_nav_accounts: 'Konten',
+    settings_accounts_caldav: 'CalDAV-Konten',
+    settings_accounts_local: 'Lokale Kalender',
+    settings_accounts_ical: 'iCal-Abonnements',
+    settings_accounts_google: 'Google-Konten',
+    settings_no_caldav_accounts: 'Keine CalDAV-Konten',
+    settings_no_local_cals: 'Keine lokalen Kalender',
+    settings_no_ical_subs: 'Keine Abonnements',
+    settings_no_google_accounts: 'Keine Google-Konten',
+    confirm_caldav_disconnect: 'CalDAV-Konto wirklich trennen?',
+    caldav_disconnected: 'CalDAV-Konto getrennt',
 
     // User management
     users_add: 'Benutzer hinzufügen', users_is_admin: 'Administrator',
@@ -269,6 +280,17 @@ const translations = {
     settings_hidden_cals: 'Hidden calendars',
     settings_no_hidden_cals: 'No hidden calendars',
     settings_no_google: 'No Google accounts connected',
+    settings_nav_accounts: 'Accounts',
+    settings_accounts_caldav: 'CalDAV Accounts',
+    settings_accounts_local: 'Local Calendars',
+    settings_accounts_ical: 'iCal Subscriptions',
+    settings_accounts_google: 'Google Accounts',
+    settings_no_caldav_accounts: 'No CalDAV accounts',
+    settings_no_local_cals: 'No local calendars',
+    settings_no_ical_subs: 'No subscriptions',
+    settings_no_google_accounts: 'No Google accounts',
+    confirm_caldav_disconnect: 'Really disconnect CalDAV account?',
+    caldav_disconnected: 'CalDAV account disconnected',
 
     // User management
     users_add: 'Add user', users_is_admin: 'Administrator',