feat(contacts): show source year instead of age in titles

A FREQ=YEARLY event has one static title, so a per-occurrence age is impossible
without heavy per-year exception rows. Replace {age} with {year} — the birth
year (or an anniversary's start year) — which is static and correct on every
occurrence, and shows everywhere (widgets, other apps, exports).

- renderSpecialDateTitle substitutes {year}; drop the age snapshot computation
  and the sync `today` parameter.
- Default templates become "{name}'s birthday ({year})" / "…anniversary ({year})".
- Rename the setting to "Show year" (prefs specialDatesShowYear) and remove the
  now-unneeded {age} snapshot disclaimer in the template editor.
- Update tests, CHANGELOG and the design-doc note.

lint + unit tests + assembleDebug green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -14,7 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   "Anniversaries" and "Other dates" calendars that stay in sync as your contacts
   change. Each is a normal local calendar, so you set its colour, visibility and
   reminders the usual way — new birthdays even start with a reminder a week before
-  *and* on the day. The title format is yours to customise (`{name}`, `{age}`).
+  *and* on the day. The title format is yours to customise (`{name}`, `{year}`).
   This is the first feature to use the contacts permission: it is requested only
   when you turn the feature on, everything stays on your device (Calendula has no
   internet access), and your contacts are only ever read, never changed. Thanks