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>

models/db.go

@@ -45,7 +45,7 @@ func InitDB(cfg *config.Config) *gorm.DB {
 	}
 
 	// Auto-migrate tables (idempotent).
-	if err := db.AutoMigrate(&User{}, &Article{}, &SiteSetting{}, &UploadConfig{}, &UploadFileType{}, &DownloadBaseURL{}, &Attachment{}, &Comment{}, &CommentConfig{}, &ArticleView{}); err != nil {
+	if err := db.AutoMigrate(&User{}, &Article{}, &SiteSetting{}, &UploadConfig{}, &UploadFileType{}, &DownloadBaseURL{}, &Attachment{}, &Comment{}, &CommentConfig{}, &ArticleView{}, &NavLink{}, &Tag{}, &ArticleTag{}); err != nil {
 		log.Fatalf("Failed to auto-migrate database: %v", err)
 	}