feat: initial blog engine with multi-language, profile, and role system

- Gin + GORM + Tailwind CSS blog engine
- OS-aware config (Linux /etc/blog_go/, Windows ./win/etc/blog_go/)
- SQLite by default, MySQL support via config
- Auto-migrate database + seed admin user on first run
- Session-based auth (login/logout)
- i18n: Chinese/English with auto-detect + manual switch
- Avatar dropdown nav with click-to-toggle menu
- Profile page: avatar upload, edit info, change password
- Role system: admin (full access) and author (profile only)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

models/db.go

@@ -0,0 +1,86 @@
+package models
+
+import (
+	"log"
+	"os"
+	"path/filepath"
+
+	"go_blog/config"
+
+	"gorm.io/driver/mysql"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
+
+	"github.com/glebarez/sqlite"
+)
+
+// DB is the global database connection, initialized by InitDB.
+var DB *gorm.DB
+
+// InitDB opens the database connection, runs migrations, and seeds the admin user.
+func InitDB(cfg *config.Config) *gorm.DB {
+	// Ensure the storage path exists.
+	if err := os.MkdirAll(cfg.Path, 0755); err != nil {
+		log.Fatalf("Failed to create storage directory %s: %v", cfg.Path, err)
+	}
+
+	var dialector gorm.Dialector
+
+	switch cfg.Database.Type {
+	case "mysql":
+		dsn := cfg.Database.DSN
+		if dsn == "" {
+			dsn = "root:password@tcp(127.0.0.1:3306)/blog_go?charset=utf8mb4&parseTime=True&loc=Local"
+		}
+		dialector = mysql.Open(dsn)
+	default:
+		dbPath := filepath.Join(cfg.Path, "blog.db")
+		dialector = sqlite.Open(dbPath)
+	}
+
+	db, err := gorm.Open(dialector, &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Warn),
+	})
+	if err != nil {
+		log.Fatalf("Failed to connect to database: %v", err)
+	}
+
+	// Auto-migrate the User table (idempotent).
+	if err := db.AutoMigrate(&User{}); err != nil {
+		log.Fatalf("Failed to auto-migrate database: %v", err)
+	}
+
+	// First-run seed: create admin user if no users exist.
+	var count int64
+	db.Model(&User{}).Count(&count)
+	if count == 0 {
+		admin := &User{
+			Username:    "admin",
+			DisplayName: "Administrator",
+			Gender:      "other",
+			Status:      StatusNormal,
+			Role:        RoleAdmin,
+		}
+		if err := admin.SetPassword("admin"); err != nil {
+			log.Fatalf("Failed to hash admin password: %v", err)
+		}
+		if err := db.Create(admin).Error; err != nil {
+			log.Fatalf("Failed to create admin user: %v", err)
+		}
+		log.Println("==============================================")
+		log.Println("  First run: created default admin user.")
+		log.Println("  Username: admin")
+		log.Println("  Password: admin")
+		log.Println("  Please change this password immediately!")
+		log.Println("==============================================")
+	}
+
+	// Migration fix: always set admin role on the admin user.
+	result := db.Model(&User{}).Where("username = ?", "admin").Update("role", RoleAdmin)
+	if result.RowsAffected > 0 {
+		log.Printf("Migration: set admin role for existing admin user (rows affected: %d)", result.RowsAffected)
+	}
+
+	DB = db
+	return db
+}