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>

i18n/i18n.go

@@ -122,6 +122,8 @@ var translations = map[Lang]map[string]string{
 		"article_published":    "Published",
 		"article_field_is_top":       "Pin to top",
 		"article_is_top":       "Pin to top",
+		"article_published_at": "Published Time",
+		"article_published_at_hint": "Leave blank to auto-set on publish",
 		"article_save_draft":   "Save Draft",
 		"article_save":         "Save",
 		"article_cancel":       "Cancel",
@@ -160,6 +162,7 @@ var translations = map[Lang]map[string]string{
 		"article_col_status":      "Status",
 		"article_col_published":   "Published",
 		"article_col_actions":     "Actions",
+		"article_last_updated":    "Last Updated",
 
 		// Settings (platform configuration)
 		"settings_nav":            "Platform Settings",
@@ -475,6 +478,8 @@ var translations = map[Lang]map[string]string{
 		"article_published":    "已发布",
 		"article_field_is_top":       "置顶",
 		"article_is_top":       "置顶",
+		"article_published_at": "发布时间",
+		"article_published_at_hint": "留空则在发布时自动设置",
 		"article_save_draft":   "保存草稿",
 		"article_save":         "保存",
 		"article_cancel":       "取消",
@@ -513,6 +518,7 @@ var translations = map[Lang]map[string]string{
 		"article_col_status":      "状态",
 		"article_col_published":   "发布时间",
 		"article_col_actions":     "操作",
+		"article_last_updated":    "最后更新",
 
 		// 平台设置
 		"settings_nav":            "平台设置",