feat: pre-fetch next email body to eliminate loading delay after delete (#381)

## Summary

- When viewing an email and then deleting (or archiving/moving/snoozing) it, the app navigates to the next email in the thread list.
- `getEmailBody` fetches from the network on a cache miss, causing the hourglass / loading spinner the issue describes.
- `EmailDetailNotifier` now fires a background `getEmailBody` call for the next thread's `latestEmailId` as soon as the current email finishes loading.
- `getEmailBody` already caches results in the `EmailBodies` table with a 7-day TTL, so by the time the user triggers a navigation action the body is pre-warmed and renders instantly.

## What changed

`lib/di.dart` — `EmailDetailNotifier.build()` calls `_prefetchNextEmailBody` (fire-and-forget via `unawaited`) after loading the current email. The helper respects the `afterMailViewAction` user preference: if set to `showMailbox` it does nothing.

## Test plan

- [ ] Open an email, delete it — next email should appear without the spinner
- [ ] Verify the same for archive, move, and snooze actions
- [ ] Verify behaviour is unchanged when `afterMailViewAction` is set to `showMailbox`
- [ ] Verify the last email in the list still pops back to the mailbox list correctly

Closes #367

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/381

lib/di.dart

@@ -211,8 +211,32 @@ class EmailDetailNotifier extends AsyncNotifier<(Email?, EmailBody)> {
       repo.getEmailBody(_emailId),
     ]);
     unawaited(repo.setFlag(_emailId, seen: true));
+    final header = results[0] as Email?;
+    if (header != null) {
+      unawaited(_prefetchNextEmailBody(repo, header));
+    }
     return (results[0] as Email?, results[1] as EmailBody);
   }
+
+  Future<void> _prefetchNextEmailBody(
+    EmailRepository repo,
+    Email header,
+  ) async {
+    final prefs = ref.read(userPreferencesProvider).value;
+    final action =
+        prefs?.afterMailViewAction ?? AfterMailViewAction.nextMessage;
+    if (action != AfterMailViewAction.nextMessage) return;
+
+    final threads =
+        await repo.observeThreads(header.accountId, header.mailboxPath).first;
+    final currentIndex = threads.indexWhere(
+      (t) => t.emailIds.contains(_emailId),
+    );
+    if (currentIndex < 0 || currentIndex + 1 >= threads.length) return;
+
+    final nextId = threads[currentIndex + 1].latestEmailId;
+    await repo.getEmailBody(nextId);
+  }
 }
 
 final accountByIdProvider =