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/tag.go

@@ -0,0 +1,126 @@
+package models
+
+import (
+	"strings"
+	"time"
+
+	"gorm.io/gorm"
+)
+
+// Tag represents a blog post tag with multi-language support.
+type Tag struct {
+	ID        uint      `gorm:"primarykey" json:"id"`
+	NameZh    string    `gorm:"size:50;not null" json:"name_zh"`
+	NameEn    string    `gorm:"size:50;not null" json:"name_en"`
+	Slug      string    `gorm:"size:100;uniqueIndex;not null" json:"slug"`
+	Count     int       `gorm:"default:0" json:"count"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+// TableName overrides the default GORM table name.
+func (Tag) TableName() string {
+	return "tags"
+}
+
+// Name returns the tag name for the specified language.
+func (t *Tag) Name(lang string) string {
+	if lang == "zh" {
+		return t.NameZh
+	}
+	return t.NameEn
+}
+
+// generateTagSlug creates a URL-friendly slug from tag name.
+func generateTagSlug(name string) string {
+	slug := strings.ToLower(strings.TrimSpace(name))
+	slug = strings.ReplaceAll(slug, " ", "-")
+	slug = strings.ReplaceAll(slug, "_", "-")
+	return slug
+}
+
+// FindOrCreateTag finds a tag by name or creates it if it doesn't exist.
+// If both nameZh and nameEn are provided, it uses them; otherwise uses the same name for both languages.
+func FindOrCreateTag(db *gorm.DB, nameZh, nameEn string) (*Tag, error) {
+	nameZh = strings.TrimSpace(nameZh)
+	nameEn = strings.TrimSpace(nameEn)
+
+	// If only one name is provided, use it for both languages
+	if nameZh == "" && nameEn != "" {
+		nameZh = nameEn
+	} else if nameEn == "" && nameZh != "" {
+		nameEn = nameZh
+	}
+
+	if nameZh == "" {
+		return nil, nil
+	}
+
+	slug := generateTagSlug(nameZh)
+
+	var tag Tag
+	err := db.Where("slug = ?", slug).First(&tag).Error
+	if err == nil {
+		return &tag, nil
+	}
+
+	if err != gorm.ErrRecordNotFound {
+		return nil, err
+	}
+
+	// Create new tag
+	tag = Tag{
+		NameZh: nameZh,
+		NameEn: nameEn,
+		Slug:   slug,
+		Count:  0,
+	}
+
+	if err := db.Create(&tag).Error; err != nil {
+		return nil, err
+	}
+
+	return &tag, nil
+}
+
+// GetAllTags returns all tags ordered by count descending.
+func GetAllTags(db *gorm.DB) ([]Tag, error) {
+	var tags []Tag
+	err := db.Order("count DESC, name_zh ASC").Find(&tags).Error
+	return tags, err
+}
+
+// GetTagBySlug returns a tag by its slug.
+func GetTagBySlug(db *gorm.DB, slug string) (*Tag, error) {
+	var tag Tag
+	err := db.Where("slug = ?", slug).First(&tag).Error
+	if err != nil {
+		return nil, err
+	}
+	return &tag, nil
+}
+
+// UpdateTagCount recalculates the article count for a tag.
+func UpdateTagCount(db *gorm.DB, tagID uint) error {
+	var count int64
+	db.Table("article_tags").Where("tag_id = ?", tagID).Count(&count)
+	return db.Model(&Tag{}).Where("id = ?", tagID).Update("count", count).Error
+}
+
+// UpdateAllTagCounts recalculates article counts for all tags.
+func UpdateAllTagCounts(db *gorm.DB) error {
+	// Get all tags
+	var tags []Tag
+	if err := db.Find(&tags).Error; err != nil {
+		return err
+	}
+
+	// Update count for each tag
+	for _, tag := range tags {
+		if err := UpdateTagCount(db, tag.ID); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}