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>

models/schema/admin.py

@@ -172,6 +172,14 @@ class AdminSettingResponse(BaseModel):
     model_config = {"from_attributes": True}
 
 
+class AdminSettingDefaultResponse(BaseModel):
+    """Response when returning a default value for non-existent setting."""
+
+    key: str
+    value: str
+    exists: bool = False
+
+
 class AdminSettingUpdate(BaseModel):
     """Update admin setting value."""