feat: add favicon settings to site configuration

- Add Favicon field to SiteSetting model with FaviconIsURL() helper
- Implement favicon upload and management in site settings page
- Support both external URL and local file upload for favicon
- Add favicon display in page header with automatic URL detection
- Add i18n translations for favicon settings (EN/ZH)
- Update middleware and helpers to pass favicon data to templates
- Include migration script for existing databases

Users can now customize their site's favicon from /admin/settings/site
by either providing an external URL or uploading a local image file
(recommended formats: .ico, .png, .svg, 16x16 or 32x32 pixels).

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

models/site_setting.go

@@ -8,6 +8,7 @@ import "time"
 type SiteSetting struct {
 	ID            uint      `gorm:"primarykey" json:"id"`
 	Logo          string    `gorm:"size:512" json:"logo"`            // local filename (served under /uploads/logos) OR a full URL
+	Favicon       string    `gorm:"size:512" json:"favicon"`         // local filename (served under /uploads/logos) OR a full URL
 	LogoTextZh    string    `gorm:"size:255" json:"logo_text_zh"`    // top-left title (zh)
 	LogoTextEn    string    `gorm:"size:255" json:"logo_text_en"`    // top-left title (en)
 	HeaderTextZh   string    `gorm:"size:512" json:"header_text_zh"`   // header banner text (zh)
@@ -36,6 +37,15 @@ func (s *SiteSetting) LogoIsURL() bool {
 	return len(s.Logo) >= 4 && (s.Logo[:4] == "http")
 }
 
+// FaviconIsURL reports whether the favicon value is an external URL rather than a
+// local filename.
+func (s *SiteSetting) FaviconIsURL() bool {
+	if s == nil || s.Favicon == "" {
+		return false
+	}
+	return len(s.Favicon) >= 4 && (s.Favicon[:4] == "http")
+}
+
 // LogoText returns the title for the given language code, falling back to the
 // other language when the requested one is empty.
 func (s *SiteSetting) LogoText(lang string) string {