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>

CHANGELOG.md

@@ -7,6 +7,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Added
+- Share a single event as an `.ics` file from the event detail screen — hands a
+  standard calendar file to any app via the system share sheet.
+- Back up your local (device-only) calendars: Settings → Calendars → Export as
+  `.ics` file writes every event of your on-device calendars to a file you
+  choose. Local calendars aren't synced anywhere, so this is their only backup.
+  (Importing `.ics` files back in lands in the next update.)
+
+_Note: new events now carry a unique identifier so a future `.ics` import can
+recognise them and avoid duplicates._
+
 ## [2.6.0] — 2026-06-18
 
 ### Added