Show birth year instead of age in birthday event titles

Title is now "🎂 Name (BirthYear)" instead of "🎂 Name (Age)".

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/birthdays/birthdays.go

@@ -115,12 +115,14 @@ func OccurrenceDate(c Contact, year int, loc *time.Location) (time.Time, bool) {
 	return occ, true
 }
 
-// Summary formats c's virtual event title for the given year:
-// "🎂 Name", or "🎂 Name (Age)" if c's birth year is known.
+// Summary formats c's virtual event title: "🎂 Name", or
+// "🎂 Name (BirthYear)" if c's birth year is known. year is unused for the
+// title itself (kept for API stability/future use) since the birth year is
+// shown as-is rather than a computed age.
 func Summary(c Contact, year int) string {
 	s := "🎂 " + c.Name
 	if c.Year > 0 {
-		s += fmt.Sprintf(" (%d)", year-c.Year)
+		s += fmt.Sprintf(" (%d)", c.Year)
 	}
 	return s
 }