Introduce CSS overrides to the platform

🔍 Problem Statement

Currently, there is no way for clients to apply custom CSS to the default Pelcro UI. This limits their ability to tailor the look and feel of their site to match their brand, resulting in a rigid visual experience that requires workarounds or direct platform changes to address styling needs.

💡 User Story

As an admin, I want to add custom CSS through the Settings > Design, so that my custom styles are applied to the site and returned via the site endpoint — allowing me to fully brand and style the Pelcro UI without requiring developer intervention.

🎯 Definition of Done (DoD)

A feature is done when:

1. Upgrade the Design settings page to use preline templates.

✔️ Given a client is in Settings > Design, when they enter CSS in the new "Custom CSS" input field and save, then the CSS is persisted and associated with their site configuration.

✔️ Given the site endpoint is called, when the response is returned, then it includes the custom CSS field with the client's saved styles.

✔️ This change will impact the UI (Settings > Design page) and the API (site endpoint response), specifically the design settings form and the site configuration payload.

✔️ This solution will include the following limitations: CSS is applied as-is with no validation or sanitization beyond basic input handling; conflicting styles may override platform defaults unpredictably; no live preview of CSS changes in the settings panel.