Closes#373
## Summary
- **Schema v38**: two new columns on `user_preferences` — `prefetch_mode` (default `wifiOnly`) and `body_cache_limit_mb` (default 100 MB).
- **`BodyCacheService`**: queries for emails that have no cached body, fetches them newest-first in batches of 20, and evicts the oldest cached bodies when the configured size limit is exceeded.
- **Separate WorkManager task** (`si_bg_prefetch`): runs hourly with `NetworkType.unmetered` (Wi-Fi) or `NetworkType.connected` (any) depending on the user's choice. The task is cancelled when prefetch is disabled.
- **App startup**: reads the stored preference from the DB and re-registers the WorkManager task with the correct constraint.
- **Preferences screen**: radio group for prefetch mode (Wi-Fi only / Any network / Disabled) and a dropdown for cache size limit (50 / 100 / 200 / 500 MB).
## What is NOT downloaded
Binary attachments are never fetched — `getEmailBody()` stores only `textBody` and `htmlBody`. The cache size limit + per-run batch cap (20 emails) keep storage bounded even on large mailboxes.
## Test plan
- [x] `task analyze` — no issues
- [x] `task test` — all 492 tests pass (incl. updated migration_test.dart for v38)
Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/400
On some Android devices (e.g. Android S1RXS32.50-13-25) the WorkManager
platform channel fails to connect at startup, throwing
PlatformException(channel-error, ...). registerBackgroundSync() now catches
PlatformException and MissingPluginException (plus any other unexpected
failure) and silently disables background sync rather than crashing the app.
Test added: test/unit/background_sync_test.dart verifies the function
completes without throwing in the unit-test environment (where the native
plugin is absent).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
