fix: 发布产物自包含数据库迁移资源，修复 macOS DMG 安装后无法启动

使用 go:embed 嵌入迁移 SQL 到二进制，移除 runtime.Caller 源码路径依赖，
server 和 desktop 发布产物均可在无源码目录环境下完成数据库初始化和迁移。

backend/internal/database/database_test.go

@@ -1,10 +1,13 @@
 package database
 
 import (
+	"io/fs"
+	"os"
 	"path/filepath"
 	"testing"
 
 	"nex/backend/internal/config"
+	"nex/backend/migrations"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -76,3 +79,87 @@ func TestBuildDSN_EmptyPassword(t *testing.T) {
 	dsn := BuildDSN(cfg)
 	assert.Equal(t, "root:@tcp(localhost:3306)/nex?charset=utf8mb4&parseTime=true&loc=Local", dsn)
 }
+
+func TestInit_SQLite_AnyCWD(t *testing.T) {
+	dir := t.TempDir()
+
+	origDir, err := os.Getwd()
+	if err == nil {
+		defer func() {
+			if chdirErr := os.Chdir(origDir); chdirErr != nil {
+				t.Logf("无法恢复工作目录: %v", chdirErr)
+			}
+		}()
+	}
+	if chdirErr := os.Chdir(dir); chdirErr != nil {
+		t.Skipf("无法切换到临时目录: %v", chdirErr)
+	}
+
+	cfg := &config.DatabaseConfig{
+		Driver:          "sqlite",
+		Path:            filepath.Join(dir, "test.db"),
+		MaxIdleConns:    5,
+		MaxOpenConns:    10,
+		ConnMaxLifetime: 0,
+	}
+
+	zapLogger := zap.NewNop()
+	db, err := Init(cfg, zapLogger)
+	require.NoError(t, err)
+	require.NotNil(t, db)
+	defer Close(db)
+
+	sqlDB, err := db.DB()
+	require.NoError(t, err)
+	require.NotNil(t, sqlDB)
+}
+
+func TestForDriverDialect_SQLite(t *testing.T) {
+	require.NoError(t, testMigrateWithDriver(t, "sqlite"))
+}
+
+func TestForDriverDialect_MySQL(t *testing.T) {
+	dialect, fsys, err := migrations.ForDriver("mysql")
+	require.NoError(t, err)
+	assert.Equal(t, "mysql", string(dialect))
+	entries, fsErr := fs.ReadDir(fsys, ".")
+	require.NoError(t, fsErr)
+	assert.NotEmpty(t, entries, "MySQL 迁移资源应至少包含一个文件")
+}
+
+func TestForDriverDialect_Invalid(t *testing.T) {
+	dir := t.TempDir()
+	cfg := &config.DatabaseConfig{
+		Driver:          "postgres",
+		Path:            filepath.Join(dir, "test.db"),
+		MaxIdleConns:    5,
+		MaxOpenConns:    10,
+		ConnMaxLifetime: 0,
+	}
+
+	zapLogger := zap.NewNop()
+	_, err := Init(cfg, zapLogger)
+	assert.Error(t, err, "非法 driver 应返回错误")
+	assert.Contains(t, err.Error(), "不支持的数据库驱动")
+}
+
+func testMigrateWithDriver(t *testing.T, driver string) error {
+	t.Helper()
+
+	dir := t.TempDir()
+	cfg := &config.DatabaseConfig{
+		Driver:          driver,
+		Path:            filepath.Join(dir, "test.db"),
+		MaxIdleConns:    5,
+		MaxOpenConns:    10,
+		ConnMaxLifetime: 0,
+	}
+
+	zapLogger := zap.NewNop()
+	db, err := Init(cfg, zapLogger)
+	if err != nil {
+		return err
+	}
+	Close(db)
+	return nil
+}