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>

handlers/auth.go

@@ -0,0 +1,64 @@
+package handlers
+
+import (
+	"net/http"
+
+	"github.com/gin-contrib/sessions"
+	"github.com/gin-gonic/gin"
+	"gorm.io/gorm"
+
+	"go_blog/models"
+)
+
+// LoginPage renders the login form.
+func LoginPage() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		tr := getTr(c)
+		data := DefaultData(c)
+		data["Title"] = tr["page_login"]
+		if c.Query("error") == "1" {
+			data["Error"] = tr["login_error"]
+		}
+		c.HTML(http.StatusOK, "login", data)
+	}
+}
+
+// Login processes the login form submission.
+func Login(db *gorm.DB) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		username := c.PostForm("username")
+		password := c.PostForm("password")
+
+		var user models.User
+		if err := db.Where("username = ?", username).First(&user).Error; err != nil {
+			c.Redirect(http.StatusFound, "/login?error=1")
+			return
+		}
+
+		if !user.CheckPassword(password) {
+			c.Redirect(http.StatusFound, "/login?error=1")
+			return
+		}
+
+		// Create session.
+		session := sessions.Default(c)
+		session.Set("user_id", user.ID)
+		session.Set("username", user.Username)
+		if err := session.Save(); err != nil {
+			c.String(http.StatusInternalServerError, "Failed to save session")
+			return
+		}
+
+		c.Redirect(http.StatusFound, "/admin")
+	}
+}
+
+// Logout clears the session and redirects home.
+func Logout() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		session := sessions.Default(c)
+		session.Clear()
+		session.Save()
+		c.Redirect(http.StatusFound, "/")
+	}
+}