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>

templates/layouts/base.html

@@ -5,6 +5,13 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>{{.Title}} - {{index .Tr "site_title"}}</title>
+    {{if .SiteFavicon}}
+        {{if .SiteFaviconIsURL}}
+            <link rel="icon" href="{{.SiteFavicon}}">
+        {{else}}
+            <link rel="icon" href="/uploads/logos/{{.SiteFavicon}}">
+        {{end}}
+    {{end}}
     <link rel="alternate" type="application/rss+xml" title="RSS Feed" href="/rss">
     <script src="https://cdn.tailwindcss.com"></script>
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/cropperjs/1.6.2/cropper.min.css">