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/AndroidManifest.xml

@@ -112,6 +112,19 @@
             </intent-filter>
         </receiver>
 
+        <!-- Hands .ics files we stage in the cache to other apps via a content
+             Uri (single-event share). Authority tracks applicationId so the
+             debug suffix doesn't break getUriForFile. -->
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.fileprovider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/file_paths" />
+        </provider>
+
         <!-- Persists the per-app language (M4) on API < 33, where the platform
              per-app-languages API is unavailable. On 33+ this is a no-op. -->
         <service