feat(backup): automatic periodic .ics export of local calendars (#8)

Scheduled one-way export of local calendars to a user-chosen folder,
overwriting calendula-backup.ics each run — the manual backup, automated.
Not a sync; stays INTERNET-free (provider reads + local file write).

- WorkManager periodic job (deps: work-runtime-ktx, documentfile); worker
  pulls collaborators via a Hilt @EntryPoint, so no custom WorkerFactory
  wiring. First periodic run is delayed one interval so it can't race the
  immediate "run now" feedback run; the writer also overwrites the canonical
  file and cleans up any "(1)" duplicates from earlier races.
- SettingsPrefs: enabled, interval (minutes, floored at 30), folder Uri,
  last-run status; persisted SAF write grant.
- UI in Calendars > Backup: toggle, folder picker, amount+unit interval
  dialog, last-run status line. Notifies after repeated failures.

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

app/build.gradle.kts

@@ -152,6 +152,9 @@ dependencies {
 
     implementation(libs.androidx.datastore.preferences)
 
+    implementation(libs.androidx.work.runtime.ktx)
+    implementation(libs.androidx.documentfile)
+
     implementation(libs.androidx.glance.appwidget)
     implementation(libs.androidx.glance.material3)