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/index.html

@@ -391,7 +391,7 @@
     <div class="settings-page-body">
       <nav class="settings-nav">
         <button class="settings-nav-btn active" data-panel="general" data-i18n="settings_nav_appearance">Darstellung</button>
-        <button class="settings-nav-btn" data-panel="google" data-i18n="settings_nav_google">Google Konten</button>
+        <button class="settings-nav-btn" data-panel="accounts" data-i18n="settings_nav_accounts">Konten</button>
         <button class="settings-nav-btn hidden" data-panel="users" id="settings-nav-users" data-i18n="settings_nav_users">Benutzerverwaltung</button>
       </nav>
 
@@ -486,10 +486,29 @@
           <div id="hidden-cals-list"><span style="font-size:13px;color:var(--text-3)" data-i18n="settings_no_hidden_cals">Keine ausgeblendeten Kalender</span></div>
         </div>
 
-        <!-- Google Konten -->
-        <div class="settings-panel" id="settings-panel-google">
-          <h4 class="panel-title" data-i18n="settings_nav_google">Google Konten</h4>
-          <div id="google-accounts-list"><span style="font-size:13px;color:var(--text-3)" data-i18n="settings_no_google">Keine Google-Konten verbunden</span></div>
+        <!-- Konten (CalDAV, Lokal, iCal, Google) -->
+        <div class="settings-panel" id="settings-panel-accounts">
+          <h4 class="panel-title" data-i18n="settings_nav_accounts">Konten</h4>
+
+          <div class="accounts-section">
+            <div class="accounts-section-heading" data-i18n="settings_accounts_caldav">CalDAV-Konten</div>
+            <div id="accounts-caldav-list"><span class="accounts-section-empty" data-i18n="settings_no_caldav_accounts">Keine CalDAV-Konten</span></div>
+          </div>
+
+          <div class="accounts-section">
+            <div class="accounts-section-heading" data-i18n="settings_accounts_local">Lokale Kalender</div>
+            <div id="accounts-local-list"><span class="accounts-section-empty" data-i18n="settings_no_local_cals">Keine lokalen Kalender</span></div>
+          </div>
+
+          <div class="accounts-section">
+            <div class="accounts-section-heading" data-i18n="settings_accounts_ical">iCal-Abonnements</div>
+            <div id="accounts-ical-list"><span class="accounts-section-empty" data-i18n="settings_no_ical_subs">Keine Abonnements</span></div>
+          </div>
+
+          <div class="accounts-section">
+            <div class="accounts-section-heading" data-i18n="settings_accounts_google">Google-Konten</div>
+            <div id="google-accounts-list"><span class="accounts-section-empty" data-i18n="settings_no_google_accounts">Keine Google-Konten</span></div>
+          </div>
         </div>
 
         <!-- Benutzerverwaltung -->