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>

templates/admin/article_create.html

@@ -77,6 +77,14 @@
             </table>
         </div>
 
+        <!-- Published At -->
+        <div>
+            <label class="block text-sm font-medium text-gray-700 mb-1">{{index .Tr "article_published_at"}}</label>
+            <input type="datetime-local" name="published_at" value="{{.FormPublishedAt}}"
+                class="w-full px-4 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-blue-500 focus:border-blue-500 outline-none transition-colors">
+            <p class="text-xs text-gray-400 mt-1">{{index .Tr "article_published_at_hint"}}</p>
+        </div>
+
         <!-- IsTop -->
         <div class="flex items-center gap-2">
             <input type="checkbox" name="is_top" value="1" id="isTopCheckbox" {{if .FormIsTop}}checked{{end}}