fix(mail): restore IMAP SOCKS5 dial timeout behavior

Preserve the 30s dial timeout semantics for proxy connections by using a timeout-backed forward dialer, and add a regression test to lock this behavior.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/mail/imap.go

@@ -37,6 +37,18 @@ type IMAPWatcher struct {
 	OnReceived func(task *database.Task)
 }
 
+const imapDialTimeout = 30 * time.Second
+
+type timeoutDialer struct {
+	timeout time.Duration
+}
+
+func (d timeoutDialer) Dial(network, address string) (net.Conn, error) {
+	return (&net.Dialer{Timeout: d.timeout}).Dial(network, address)
+}
+
+var socks5DialerFactory = proxy.SOCKS5
+
 // NewIMAPWatcher creates a new IMAP watcher.
 func NewIMAPWatcher(cfg *config.Config, db *gorm.DB) *IMAPWatcher {
 	return &IMAPWatcher{
@@ -177,7 +189,7 @@ func dialTLSViaSOCKS5(addr, proxyURL string, options *imapclient.Options) (*imap
 		auth = &proxy.Auth{User: u.User.Username(), Password: pw}
 	}
 
-	dialer, err := proxy.SOCKS5("tcp", u.Host, auth, proxy.Direct)
+	dialer, err := socks5DialerFactory("tcp", u.Host, auth, timeoutDialer{timeout: imapDialTimeout})
 	if err != nil {
 		return nil, fmt.Errorf("create socks5 dialer: %w", err)
 	}