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>

handlers/helpers.go

@@ -19,6 +19,8 @@ func DefaultData(c *gin.Context) gin.H {
 	siteSetting, _ := c.Get("site_setting")
 	siteLogo, _ := c.Get("site_logo")
 	siteLogoIsURL, _ := c.Get("site_logo_is_url")
+	siteFavicon, _ := c.Get("site_favicon")
+	siteFaviconIsURL, _ := c.Get("site_favicon_is_url")
 	siteLogoText, _ := c.Get("site_logo_text")
 	siteHeaderText, _ := c.Get("site_header_text")
 	siteHomeWelcome, _ := c.Get("site_home_welcome")
@@ -37,6 +39,8 @@ func DefaultData(c *gin.Context) gin.H {
 		"SiteSetting":      siteSetting,
 		"SiteLogo":         siteLogo,
 		"SiteLogoIsURL":    siteLogoIsURL,
+		"SiteFavicon":      siteFavicon,
+		"SiteFaviconIsURL": siteFaviconIsURL,
 		"SiteLogoText":     siteLogoText,
 		"SiteHeaderText":   siteHeaderText,
 		"SiteHomeWelcome":  siteHomeWelcome,