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

ci/main.go

@@ -565,6 +565,16 @@ func (m *Ci) TestSyncReliability(ctx context.Context) (string, error) {
 		Stdout(ctx)
 }
 
+// ChaosMonkeyBackend runs random IMAP/SMTP operations against Stalwart to surface crashes.
+func (m *Ci) ChaosMonkeyBackend(ctx context.Context) (string, error) {
+	return m.WithStalwart(m.setup(m.backendSrc())).
+		WithExec([]string{"/bin/bash", "-c",
+			`tmp=$(mktemp); trap 'rm -f "$tmp"' EXIT; ` +
+				`flutter test test/backend/chaos_monkey_test.dart --reporter expanded --concurrency=1 --no-pub >"$tmp" 2>&1 || { cat "$tmp"; exit 1; }; ` +
+				`grep -E '^All [0-9]+ tests passed' "$tmp" || tail -1 "$tmp"`}).
+		Stdout(ctx)
+}
+
 // Check runs the full check suite.
 func (m *Ci) Check(ctx context.Context) (string, error) {
 	ctx, cancel := context.WithTimeout(ctx, 30*time.Minute)