fix: resolve settings page icon and 404 errors

- Change icon from 'envelope' to 'mail' (envelope not in icons.js)
- Add default query param to GET /admin/settings/{key} endpoint
- Return AdminSettingDefaultResponse instead of 404 when default provided
- Update loadShippingSettings() to use default param for carrier settings

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/settings.html

@@ -20,7 +20,7 @@
 {% call tabs_nav() %}
     {{ tab_button('display', 'Display', icon='view-grid') }}
     {{ tab_button('logging', 'Logging', icon='document-text') }}
-    {{ tab_button('email', 'Email', icon='envelope') }}
+    {{ tab_button('email', 'Email', icon='mail') }}
     {{ tab_button('shipping', 'Shipping', icon='truck') }}
     {{ tab_button('system', 'System', icon='cog') }}
     {{ tab_button('security', 'Security', icon='shield-check') }}