Adds default patterns view setting

[ronalfy]

Introduces a new user-specific setting to define the default filter for the Patterns Library.

This enhancement allows users to:

• Set their preferred initial view (e.g., "All Local Patterns", "Synced Patterns") when accessing the Patterns Library without specific URL filters.
• Improve their workflow by automatically applying a chosen filter, making it quicker to find relevant patterns.

The changes involve:

• Adding a new UI control in the admin settings for users to select their default view.
• Storing and retrieving this preference via site-scoped user meta.
• Implementing logic to dynamically apply the saved default view to the Patterns Library when no explicit filters are present in the URL.
• Introducing utility functions to manage allowed view slugs, sanitize inputs, and merge view presets into query arguments.