Add birthday feature (web)

- views render display_title (age) and a cake icon for is_birthday events
  (month/week/agenda + quarter tooltip) via shared eventTitle/birthdayIconSvg
- create split-button caret opens a menu: new event / new birthday
- new birthday modal: name + date + "year unknown" + target birthday calendar,
  saved as an all-day FREQ=YEARLY local event with birth_year
- local-calendar modal gains a "birthday calendar" toggle + "notify N days before"
- i18n de/en strings

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

frontend/js/views/quarter.js

@@ -1,4 +1,4 @@
-import { isToday, isPast, dayOfWeek } from '../utils.js';
+import { isToday, isPast, dayOfWeek, eventTitle } from '../utils.js';
 import { t } from '../i18n.js';
 
 export function renderQuarter(container, currentDate, events, onDayClick, onEventClick, weekStartDay = 'monday') {
@@ -64,7 +64,7 @@ export function renderQuarter(container, currentDate, events, onDayClick, onEven
         const dots = cellEvs.slice(0, 3).map(ev => {
           const color = ev.color || ev.calendarColor || '#4285f4';
           const pastCls = isPast(ev) ? 'past' : '';
-          return `<span class="qtr-dot ${pastCls}" style="background:${color}" title="${escAttr(ev.title)}" data-id="${ev.id}" data-url="${escAttr(ev.url || '')}"></span>`;
+          return `<span class="qtr-dot ${pastCls}" style="background:${color}" title="${escAttr(eventTitle(ev))}" data-id="${ev.id}" data-url="${escAttr(ev.url || '')}"></span>`;
         }).join('');
         const moreDot = cellEvs.length > 3
           ? `<span class="qtr-dot-more">+${cellEvs.length - 3}</span>`