Rework birthdays to a single calendar + UI fixes (web)

- one dedicated birthday calendar per user; enable it in Settings > Calendars
  (or from the New-birthday dialog) — it then shows in the sidebar like any
  calendar (colour/visibility)
- New-birthday dialog: no target picker; day/month selects + a year field that
  hides when "year unknown"; fixed the year-unknown checkbox layout
- create split-button is now one seamless pill; the caret opens the menu
- Settings > Calendars gains a Birthdays section (enable + notify-days-before)
- removed the birthday toggle from the generic new-calendar modal
- profile settings: right-align the section save button for consistency
- bump service-worker cache to v25 so assets refresh

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

frontend/sw.js

@@ -7,7 +7,7 @@
 // the entry HTML / version files). New releases take effect on the next
 // reload, no manual SW unregister required.
 
-const CACHE_VERSION  = 'calendarr-v24';
+const CACHE_VERSION  = 'calendarr-v25';
 const OFFLINE_SHELL  = ['/', '/index.html'];
 
 self.addEventListener('install', event => {