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>

i18n/i18n.go

@@ -170,6 +170,11 @@ var translations = map[Lang]map[string]string{
 		"settings_logo_url":       "Logo URL (external link)",
 		"settings_logo_upload":    "Or upload a logo image",
 		"settings_logo_clear":     "Remove current logo",
+		"settings_favicon":        "Favicon",
+		"settings_favicon_url":    "Favicon URL (external link)",
+		"settings_favicon_current": "Current favicon",
+		"settings_favicon_hint":   "Recommended: .ico, .png or .svg format, 32x32 or 16x16 pixels",
+		"settings_favicon_clear":  "Remove current favicon",
 		"settings_logo_text_zh":   "Site Title (Chinese)",
 		"settings_logo_text_en":   "Site Title (English)",
 		"settings_header_text_zh": "Header Banner Text (Chinese)",
@@ -518,6 +523,11 @@ var translations = map[Lang]map[string]string{
 		"settings_logo_url":       "Logo 链接（外链地址）",
 		"settings_logo_upload":    "或上传 Logo 图片",
 		"settings_logo_clear":     "移除当前 Logo",
+		"settings_favicon":        "网站图标（Favicon）",
+		"settings_favicon_url":    "Favicon 链接（外链地址）",
+		"settings_favicon_current": "当前 Favicon",
+		"settings_favicon_hint":   "推荐：.ico、.png 或 .svg 格式，32x32 或 16x16 像素",
+		"settings_favicon_clear":  "移除当前 Favicon",
 		"settings_logo_text_zh":   "站点标题（中文）",
 		"settings_logo_text_en":   "站点标题（英文）",
 		"settings_header_text_zh": "顶部横幅文本（中文）",