feat(detail): duplicate an event as a new one (#52)

Add a Duplicate action to the event detail top bar that opens the shared
event form seeded with a copy of the event as a new, unsaved event, so a
non-recurring event can be re-created with just the day and time changed
instead of re-entering every field.

The copy reuses the existing prefilled-create overlay (createEvent), so it
becomes an independent event with the default reminder applied. Recurrence
is dropped — a duplicate is a single event; the edit form still exposes a
recurrence picker for anyone who wants a series. The occurrence's own times
carry over unchanged. Duplicate is offered for any loaded event, including
read-only ones (WebCal, birthdays): the source calendar is kept only when
it's writable, otherwise the copy resolves to the first writable calendar.

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

app/src/main/res/values/strings.xml

@@ -49,6 +49,7 @@
     <string name="event_detail_edit">Edit</string>
     <string name="event_detail_delete">Delete</string>
     <string name="event_detail_share">Share</string>
+    <string name="event_detail_duplicate">Duplicate</string>
     <string name="event_share_chooser_title">Share event</string>
     <string name="event_share_failed">Couldn\'t share this event.</string>
     <string name="event_delete_title">Delete event?</string>