feat: add custom publish time and last updated time display

- Add custom publish time field to article create/edit forms
  - Support datetime-local input for manual time setting
  - Auto-set on first publish if left blank
  - Works for both admin and user article forms

- Display last updated time on article detail page
  - Show both published time and last updated time
  - Auto-maintained by GORM on each update
  - Format: YYYY-MM-DD HH:MM

- Add i18n support for new fields
  - article_published_at: Published Time / 发布时间
  - article_published_at_hint: Leave blank to auto-set on publish / 留空则在发布时自动设置
  - article_last_updated: Last Updated / 最后更新

- Update handlers and templates
  - handlers/article.go: Add parsePublishedAt/formatPublishedAt functions
  - handlers/home.go: Add formatUpdateTime function
  - handlers/my_articles.go: Support custom publish time in user articles
  - templates: Add datetime-local inputs and time display

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

handlers/home.go

@@ -191,6 +191,7 @@ func renderArticleDetail(c *gin.Context, db *gorm.DB, article *models.Article, f
 	data["Article"] = article
 	data["AuthorName"] = authorName
 	data["PublishedAt"] = formatPublishTime(article.PublishedAt)
+	data["UpdatedAt"] = formatUpdateTime(article.UpdatedAt)
 	data["Comments"] = tree
 	data["CommentConfig"] = models.GetCommentConfig()
 	data["CommentForm"] = form
@@ -209,6 +210,11 @@ func formatPublishTime(publishedAt *time.Time) string {
 	return ""
 }
 
+// formatUpdateTime returns the last update time as a readable string.
+func formatUpdateTime(updatedAt time.Time) string {
+	return updatedAt.Format("2006-01-02 15:04")
+}
+
 // commentFlashKey is the session key for the one-time comment notice.
 const commentFlashKey = "comment_flash"