feat: pre-fetch email bodies for offline access (#400)

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

test/unit/migration_test.dart

@@ -14,7 +14,7 @@ void main() {
   group('Migration', () {
     test('schemaVersion matches expected value', () async {
       final db = AppDatabase(NativeDatabase.memory());
-      expect(db.schemaVersion, 37);
+      expect(db.schemaVersion, 38);
       await db.close();
     });
 
@@ -420,12 +420,16 @@ void main() {
             .customSelect('SELECT count(*) FROM image_trusted_senders')
             .get();
 
+        // v38: prefetch_mode and body_cache_limit_mb columns on user_preferences.
+        expect(userPrefsColumns, contains('prefetch_mode'));
+        expect(userPrefsColumns, contains('body_cache_limit_mb'));
+
         await db.close();
         if (dbFile.existsSync()) dbFile.deleteSync();
       },
     );
 
-    test('fresh install creates all tables at schemaVersion 37', () async {
+    test('fresh install creates all tables at schemaVersion 38', () async {
       final db = AppDatabase(NativeDatabase.memory());
       await db.select(db.accounts).get();
 
@@ -485,6 +489,10 @@ void main() {
       // v37: image_trusted_senders table.
       await db.customSelect('SELECT count(*) FROM image_trusted_senders').get();
 
+      // v38: prefetch_mode and body_cache_limit_mb columns on user_preferences.
+      expect(userPrefsColumns, contains('prefetch_mode'));
+      expect(userPrefsColumns, contains('body_cache_limit_mb'));
+
       await db.close();
     });
   });

test/widget/helpers.dart

@@ -663,6 +663,12 @@ class FakeUserPreferencesRepository implements UserPreferencesRepository {
     afterMailViewAction = action;
   }
 
+  @override
+  Future<void> updatePrefetchMode(PrefetchMode mode) async {}
+
+  @override
+  Future<void> updateBodyCacheLimitMb(int mb) async {}
+
   @override
   Stream<List<String>> observeTrustedImageSenders() =>
       Stream.value(List.of(_trustedImageSenders));