fix: resolve dart analyze failures in chaos_monkey_test.dart (#458)

## Summary

Fixes CI failures introduced by PR #455 (chaos monkey backend test).

The `dart analyze --fatal-infos` step in CI was failing because `test/backend/chaos_monkey_test.dart` had:

- **`avoid_print`** (5 instances): replaced `print(...)` with `stdout.writeln(...)` — `dart:io` is already imported
- **`avoid_redundant_argument_values`**: removed redundant `''` from `_env('CHAOS_SEED', '')` since `''` is the parameter default
- **`dart format`**: applied formatter fixes (trailing commas, line wrapping for long `connectToServer` calls)

## Verification

```
$ nix develop --command bash -c "fvm dart analyze --fatal-infos"
Analyzing 456...
No issues found!

$ nix develop --command bash -c "fvm dart format --output=none --set-exit-if-changed test/backend/chaos_monkey_test.dart"
Formatted 1 file (0 changed) in 0.01 seconds.
```

Closes #456

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Thomas SharedInbox <sharedinbox@thomas-guettler.de>
Reviewed-on: https://codeberg.org/guettli/sharedinbox/pulls/458

Taskfile.yml

@@ -708,6 +708,11 @@ tasks:
     cmds:
       - fvm flutter test test/screenshot_automation_test.dart --update-goldens
 
+  chaos-monkey-backend:
+    desc: Chaos monkey — random IMAP/SMTP ops against Stalwart (via Dagger, headless)
+    cmds:
+      - timeout --kill-after=10 600 dagger call --progress=plain -q -m ci --source=. chaos-monkey-backend
+
   check:
     desc: Full check suite — unit tests first, then integration (merges coverage), then gate
     deps: [analyze, build-linux, test]