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/article.go

@@ -30,6 +30,7 @@ type Article struct {
 	Slug      string         `gorm:"uniqueIndex:idx_slug_deleted_at;size:255" json:"slug"`
 	PublishedAt *time.Time   `gorm:"index" json:"published_at"`
 	Author    User           `gorm:"foreignKey:AuthorID" json:"-"`
+	Tags      []Tag          `gorm:"many2many:article_tags;" json:"tags"`
 }
 
 // TableName overrides the default GORM table name.