feat(ics): export — share single event + back up local calendars as .ics

Branch 1 of 2 for v2.7 (the .ics topic). Adds the write side of a
hand-rolled RFC 5545 engine (zero deps, stays on kotlinx-datetime):

- domain/ics: IcsText (escape + 75-octet folding), IcsEvent model,
  IcsWriter.writeCalendar. Timezone rule: all-day VALUE=DATE, one-off
  timed UTC Z, recurring timed TZID-labelled from EVENT_TIMEZONE (no
  VTIMEZONE — import resolves TZID against the OS tz db).
- Single-event share from the detail screen (FileProvider + ACTION_SEND).
- Whole-calendar backup of the writable local calendars to a SAF file
  (Settings -> Calendars -> Export as .ics), one combined VCALENDAR.
- insertEvent now writes Events.UID_2445; legacy rows fall back to a
  stable synthesised UID at export time so a later restore won't dupe.
- EXDATE / RECURRENCE-ID overrides are deliberately skipped this pass
  (documented v1 limit; import will skip them too).

Engine + mapper unit-tested. Import (Branch 2, feat/ics-import) ships in
the same v2.7 release; no tag until both land + on-device review.

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

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

@@ -48,6 +48,9 @@
     <string name="event_detail_back">Back</string>
     <string name="event_detail_edit">Edit</string>
     <string name="event_detail_delete">Delete</string>
+    <string name="event_detail_share">Share</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>
     <string name="event_delete_body">The event is removed from your calendar and from every device it syncs to.</string>
     <string name="event_delete_recurring_title">Delete recurring event</string>
@@ -294,6 +297,15 @@
     <string name="calendars_delete_confirm_title">Delete calendar?</string>
     <string name="calendars_delete_confirm_message">\"%1$s\" and all of its events will be permanently removed from this device.</string>
     <string name="calendars_write_error">Couldn\'t save the change.</string>
+    <!-- Backup (whole-calendar .ics export) -->
+    <string name="calendars_backup_header">Backup</string>
+    <string name="calendars_backup_hint">Local calendars aren\'t synced anywhere, so export them to an .ics file to keep a copy.</string>
+    <string name="calendars_backup_action">Export as .ics file</string>
+    <string name="calendars_backup_failed">Couldn\'t export the backup.</string>
+    <plurals name="calendars_backup_done">
+        <item quantity="one">Exported %d event.</item>
+        <item quantity="other">Exported %d events.</item>
+    </plurals>
     <!-- Launcher long-press shortcuts -->
     <string name="shortcut_new_event_short">New event</string>
     <string name="shortcut_new_event_long">Create a new event</string>