Birthdays: explicit "Birthday calendar" add type + tidier table defaults (web)

- add "Geburtstagskalender" to the sidebar +calendar dropdown and a "+ Geburtstage"
  button to Settings > Calendars; both call ensureBirthdayCalendar (server enforces
  one per account) and toast "already exists" if present
- Settings birthday section: when none exists, just a hint pointing to "+ Geburtstage"
  (creation lives in the add area now, not a separate activate button)
- resizable calendar table: sensible default column widths instead of raw measured
- i18n de/en

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

frontend/index.html

@@ -206,6 +206,7 @@
               </button>
               <div class="add-cal-dropdown hidden" id="add-cal-dropdown">
                 <button data-action="local">Lokaler Kalender</button>
+                <button data-action="birthday" data-i18n="birthday_calendar_type">Geburtstagskalender</button>
                 <button data-action="caldav">CalDAV-Konto</button>
                 <button data-action="ical">iCal-URL abonnieren</button>
                 <button data-action="google">Google Kalender</button>
@@ -934,6 +935,7 @@
           <h4 class="panel-title" data-i18n="settings_nav_calendars">Kalender</h4>
           <div class="accounts-add-row">
             <button class="btn btn-secondary btn-sm" id="settings-btn-add-local">+ Lokal</button>
+            <button class="btn btn-secondary btn-sm" id="settings-btn-add-birthday" data-i18n="birthday_add_btn">+ Geburtstage</button>
             <button class="btn btn-secondary btn-sm" id="settings-btn-add-caldav">+ CalDAV</button>
             <button class="btn btn-secondary btn-sm" id="settings-btn-add-ical">+ iCal</button>
             <button class="btn btn-secondary btn-sm" id="settings-btn-add-ha">+ Home Assistant</button>