fix(calendar): expand recurring multi-day events and support YEARLY frequency

Root causes:
1. parseRRule did not strip the "RRULE:" prefix stored by the ICS parser,
   causing all recurrence rules from CalDAV sync to silently fail parsing
2. YEARLY frequency (used by birthday events) was not supported
3. expandRecurringEvents filtered instances only by start date, missing
   multi-day events that start before the view window but span into it
4. All-day recurring instances got datetime end values instead of date-only

Fixes #5 (follow-up from @tschig)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "oikos",
-  "version": "0.5.5",
+  "version": "0.5.7",
   "description": "Selbstgehosteter Familienplaner — Kalender, Aufgaben, Einkauf, Essensplan, Budget und mehr. Privat, offen, ohne Abo.",
   "main": "server/index.js",
   "engines": {