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>

gradle/libs.versions.toml

@@ -26,10 +26,14 @@ lifecycleCompose = "2.10.0"
 androidxTestRules = "1.7.0"
 # Glance: 1.1.1 is the latest stable (1.2.0 is still rc, 1.3.0 alpha).
 glance = "1.1.1"
+work = "2.10.1"
+documentfile = "1.0.1"
 
 [libraries]
 # AndroidX core
 androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }
+androidx-work-runtime-ktx = { group = "androidx.work", name = "work-runtime-ktx", version.ref = "work" }
+androidx-documentfile = { group = "androidx.documentfile", name = "documentfile", version.ref = "documentfile" }
 androidx-appcompat = { group = "androidx.appcompat", name = "appcompat", version.ref = "appcompat" }
 androidx-lifecycle-runtime-ktx = { group = "androidx.lifecycle", name = "lifecycle-runtime-ktx", version.ref = "lifecycleRuntime" }
 androidx-activity-compose = { group = "androidx.activity", name = "activity-compose", version.ref = "activityCompose" }