feat: implement tag management and search functionality

Features:
- Tag system with multi-language support (Chinese/English)
- Auto-create tags when associating with articles
- Tag filtering on homepage with visual indicators
- Full-text search across title, summary, and content
- Tag cloud sidebar showing article counts
- Search page with results display

Technical Implementation:
- Database models: Tag, ArticleTag (many-to-many)
- Tag count caching for performance
- Automatic tag slug generation
- Responsive design with mobile support
- I18n support for all new UI elements

Bug Fix:
- Fixed SQL column ambiguity error in tag filtering
- Added table prefix to ORDER BY clause in publishedArticleOrder

Routes:
- GET /search - Search results page
- GET /?tag=<slug> - Filter articles by tag
- GET /api/articles?tag=<slug> - API with tag filter support

Templates:
- Added tag input field to article create/edit form
- Added search box to homepage header
- Added tag cloud sidebar on homepage and search page
- Created new search results page template

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

templates/admin/article_create.html

@@ -53,6 +53,15 @@
                 placeholder="https://...">
         </div>
 
+        <!-- Tags -->
+        <div>
+            <label class="block text-sm font-medium text-gray-700 mb-1">{{index .Tr "article_tags"}}</label>
+            <input type="text" name="tags" value="{{.FormTags}}"
+                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"
+                placeholder="{{index .Tr "article_tags_hint"}}">
+            <p class="text-xs text-gray-400 mt-1">{{index .Tr "article_tags_hint"}}</p>
+        </div>
+
         <!-- Attachments -->
         <div>
             <label class="block text-sm font-medium text-gray-700 mb-1">{{index .Tr "article_attachments"}}</label>