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/admin/settings_site.html

@@ -34,6 +34,29 @@
             </label>
         </div>
 
+        <!-- Favicon -->
+        <div>
+            <label class="block text-sm font-semibold text-gray-700 mb-2">{{index .Tr "settings_favicon"}}</label>
+            {{if .Site.Favicon}}
+                <div class="flex items-center gap-2 mb-2">
+                    <span class="text-sm text-gray-600">{{index .Tr "settings_favicon_current"}}:</span>
+                    {{if .SiteFaviconIsURL}}
+                        <img src="{{.Site.Favicon}}" alt="favicon" class="h-8 w-8">
+                    {{else}}
+                        <img src="/uploads/logos/{{.Site.Favicon}}" alt="favicon" class="h-8 w-8">
+                    {{end}}
+                </div>
+            {{end}}
+            <input type="text" name="favicon_url" placeholder="{{index .Tr "settings_favicon_url"}}"
+                class="w-full border border-gray-300 rounded-lg px-3 py-2 mb-2" value="{{if .SiteFaviconIsURL}}{{.Site.Favicon}}{{end}}">
+            <input type="file" name="favicon" accept="image/x-icon,image/png,image/svg+xml"
+                class="block w-full text-sm text-gray-500 mb-2">
+            <p class="text-xs text-gray-500 mb-2">{{index .Tr "settings_favicon_hint"}}</p>
+            <label class="inline-flex items-center gap-2 text-sm text-gray-600">
+                <input type="checkbox" name="favicon_clear" value="1"> {{index .Tr "settings_favicon_clear"}}
+            </label>
+        </div>
+
         <!-- Title texts -->
         <div class="grid grid-cols-1 sm:grid-cols-2 gap-4">
             <div>