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/user/my_article_form.html

@@ -48,6 +48,13 @@
             <p class="mt-1 text-sm text-gray-500">{{index .Tr "article_cover_help"}}</p>
         </div>
 
+        <div>
+            <label class="block text-sm font-medium text-gray-700 mb-2">{{index .Tr "article_published_at"}}</label>
+            <input type="datetime-local" name="published_at" value="{{.FormPublishedAt}}"
+                class="w-full px-3 py-2 border border-gray-300 rounded-lg focus:ring-2 focus:ring-blue-500 focus:border-blue-500">
+            <p class="mt-1 text-sm text-gray-500">{{index .Tr "article_published_at_hint"}}</p>
+        </div>
+
         <div class="grid grid-cols-1 md:grid-cols-2 gap-6">
             <div>
                 <label class="block text-sm font-medium text-gray-700 mb-2">{{index .Tr "article_field_status"}}</label>