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>

middleware/auth.go

@@ -0,0 +1,100 @@
+package middleware
+
+import (
+	"net/http"
+
+	"github.com/gin-contrib/sessions"
+	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
+
+	"go_blog/i18n"
+	"go_blog/models"
+)
+
+// AuthRequired is middleware that protects routes. If the user is not logged in,
+// they are redirected to /login.
+func AuthRequired() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		session := sessions.Default(c)
+		userID := session.Get("user_id")
+		if userID == nil {
+			c.Redirect(http.StatusFound, "/login")
+			c.Abort()
+			return
+		}
+		c.Next()
+	}
+}
+
+// SetUserContext is global middleware that reads the session and sets
+// template-friendly context values for all pages (language, auth state, etc.).
+func SetUserContext(db *gorm.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		session := sessions.Default(c)
+
+		// --- Language detection ---
+		// Priority: query param > session > Accept-Language header > default EN
+		var lang i18n.Lang
+		queryLang := c.Query("lang")
+
+		switch queryLang {
+		case "zh":
+			lang = i18n.ZH
+		case "en":
+			lang = i18n.EN
+		case "":
+			// Try session
+			if saved, ok := session.Get("lang").(string); ok {
+				lang = i18n.Lang(saved)
+			}
+			if lang == "" {
+				// Try Accept-Language header
+				lang = i18n.DetectLang(c.GetHeader("Accept-Language"))
+			}
+		default:
+			// Unsupported language in query — fall back to English.
+			lang = i18n.EN
+		}
+
+		// Persist language in session.
+		session.Set("lang", string(lang))
+		session.Save()
+
+		// Make translations available in the Gin context.
+		c.Set("tr", i18n.T(lang))
+		c.Set("lang", string(lang))
+
+		// Set the opposite language code for the language switcher link.
+		switchLang := "zh"
+		if lang == i18n.ZH {
+			switchLang = "en"
+		}
+		c.Set("switch_lang", switchLang)
+
+		// --- Auth state ---
+		userID := session.Get("user_id")
+		isLoggedIn := false
+		var username string
+		var avatar string
+		var displayName string
+		var role string
+
+		if userID != nil {
+			isLoggedIn = true
+			var user models.User
+			if err := db.First(&user, userID).Error; err == nil {
+				username = user.Username
+				avatar = user.Avatar
+				displayName = user.DisplayName
+				role = user.Role
+			}
+		}
+
+		c.Set("is_logged_in", isLoggedIn)
+		c.Set("username", username)
+		c.Set("avatar", avatar)
+		c.Set("display_name", displayName)
+		c.Set("role", role)
+		c.Next()
+	}
+}