refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/pkg/mail/mail_test.go

@@ -0,0 +1,112 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package mail
+
+import (
+	"bufio"
+	"context"
+	"net"
+	"net/textproto"
+	"testing"
+)
+
+func TestSendMailMock(t *testing.T) {
+	// Start a mock SMTP server
+	l, err := net.Listen("tcp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("failed to start mock smtp server: %v", err)
+	}
+	defer func() { _ = l.Close() }()
+
+	port := l.Addr().(*net.TCPAddr).Port
+
+	go func() {
+		conn, err := l.Accept()
+		if err != nil {
+			return
+		}
+		defer func() { _ = conn.Close() }()
+
+		writer := bufio.NewWriter(conn)
+		reader := bufio.NewReader(conn)
+		tp := textproto.NewReader(reader)
+
+		// 220 Ready
+		_, _ = writer.WriteString("220 mock.smtp.com SMTP Ready\r\n")
+		_ = writer.Flush()
+
+		// Read HELO/EHLO
+		_, _ = tp.ReadLine()
+		_, _ = writer.WriteString("250-mock.smtp.com\r\n250 AUTH PLAIN\r\n")
+		_ = writer.Flush()
+
+		// Read AUTH PLAIN
+		_, _ = tp.ReadLine()
+		_, _ = writer.WriteString("235 Authentication successful\r\n")
+		_ = writer.Flush()
+
+		// Read MAIL FROM
+		_, _ = tp.ReadLine()
+		_, _ = writer.WriteString("250 OK\r\n")
+		_ = writer.Flush()
+
+		// Read RCPT TO
+		_, _ = tp.ReadLine()
+		_, _ = writer.WriteString("250 OK\r\n")
+		_ = writer.Flush()
+
+		// Read DATA
+		_, _ = tp.ReadLine()
+		_, _ = writer.WriteString("354 Start mail input\r\n")
+		_ = writer.Flush()
+
+		// Read body lines until dot
+		for {
+			line, err := tp.ReadLine()
+			if err != nil || line == "." {
+				break
+			}
+		}
+		_, _ = writer.WriteString("250 OK\r\n")
+		_ = writer.Flush()
+
+		// Read QUIT
+		_, _ = tp.ReadLine()
+		_, _ = writer.WriteString("221 Bye\r\n")
+		_ = writer.Flush()
+	}()
+
+	cfg := Config{
+		Host:     "127.0.0.1",
+		Port:     port,
+		Username: "test@example.com",
+		Password: "password",
+	}
+
+	err = SendMail(context.Background(), cfg, "recipient@example.com", "Test Subject", "<h1>Test Body</h1>")
+	if err != nil {
+		t.Errorf("failed to send mail: %v", err)
+	}
+}
+
+func TestSanitizeHeaderValue(t *testing.T) {
+	tests := []struct {
+		name  string
+		input string
+		want  string
+	}{
+		{"plain", "System Notification", "System Notification"},
+		{"crlf stripped", "alert\r\nBcc: attacker@example.com", "alertBcc: attacker@example.com"},
+		{"cr stripped", "a\rb", "ab"},
+		{"lf stripped", "a\nb", "ab"},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := sanitizeHeaderValue(tt.input); got != tt.want {
+				t.Errorf("sanitizeHeaderValue(%q) = %q, want %q", tt.input, got, tt.want)
+			}
+		})
+	}
+}