feat: Add defaultPinMcp configuration option to interface settings

[victorbjor]

Summary

This PR introduces a new configuration option defaultPinMcp to the interface settings in Librechat.yaml, allowing administrators to control the default pinned state of MCP servers in the chat interface across all users.

Change Type

• New feature (non-breaking change which adds functionality)

Testing

Manual Test Configuration:

• Tested with defaultPinMcp: true (default behavior maintained)
• Tested with defaultPinMcp: false (MCP servers unpinned by default)
• Verified that existing user preferences are preserved when the setting is changed
• Confirmed that the setting properly initializes the useMCPSelect hook's local storage value

Test Process:

Set defaultPinMcp: false in the interface configuration
Verify that new users see MCP servers unpinned by default
Set defaultPinMcp: true and confirm MCP servers are pinned by default
Verify that existing user preferences stored in localStorage are not affected by configuration changes

Checklist

• My code adheres to this project's style guidelines
• I have performed a self-review of my own code
• I have commented in any complex areas of my code
• My changes do not introduce new warnings
• Local unit tests pass with my changes
• My changes are backward compatible and maintain existing functionality