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/helpers.go

@@ -0,0 +1,44 @@
+package handlers
+
+import (
+	"github.com/gin-gonic/gin"
+)
+
+// DefaultData builds a base gin.H map populated with values set by the
+// SetUserContext middleware (translations, language, auth state). Handlers
+// add page-specific fields on top and pass it to c.HTML().
+func DefaultData(c *gin.Context) gin.H {
+	tr, _ := c.Get("tr")
+	isLoggedIn, _ := c.Get("is_logged_in")
+	username, _ := c.Get("username")
+	lang, _ := c.Get("lang")
+	switchLang, _ := c.Get("switch_lang")
+	avatar, _ := c.Get("avatar")
+	displayName, _ := c.Get("display_name")
+	role, _ := c.Get("role")
+
+	return gin.H{
+		"Tr":          tr,
+		"IsLoggedIn":  isLoggedIn,
+		"Username":    username,
+		"Lang":        lang,
+		"SwitchLang":  switchLang,
+		"Avatar":      avatar,
+		"DisplayName": displayName,
+		"Role":        role,
+	}
+}
+
+// getTr is a convenience helper that returns the translation map from the
+// Gin context, falling back to an empty map if not set.
+func getTr(c *gin.Context) map[string]string {
+	tr, ok := c.Get("tr")
+	if !ok {
+		return map[string]string{}
+	}
+	m, ok := tr.(map[string]string)
+	if !ok {
+		return map[string]string{}
+	}
+	return m
+}