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>

middleware/auth.go

@@ -123,6 +123,8 @@ func SetUserContext(db *gorm.DB) gin.HandlerFunc {
 		c.Set("site_setting", site)
 		c.Set("site_logo", site.Logo)
 		c.Set("site_logo_is_url", site.LogoIsURL())
+		c.Set("site_favicon", site.Favicon)
+		c.Set("site_favicon_is_url", site.FaviconIsURL())
 		c.Set("site_logo_text", site.LogoText(string(lang)))
 		c.Set("site_header_text", site.HeaderText(string(lang)))
 		c.Set("site_home_welcome", site.HomeWelcome(string(lang)))