feat: dynamic custom model IDs and flexible model mapping

[hitesharma]

Summary

This PR introduces the ability for users to dynamically add, remove, and map custom model IDs through the Web UI. It removes the reliance on a hardcoded list of supported models, allowing for easier
integration of new or specialized model variants without requiring a server restart or code changes.

Key Changes

• Dynamic Model Management: Added backend API endpoints (/api/models) and UI controls to add or delete custom model IDs that persist across sessions.
• Flexible Model Mapping: Converted the "Target Model ID" mapping in settings from a strict dropdown to a text input with autocomplete support, enabling redirection to any arbitrary model ID.
• Enhanced Validation: Updated server-side validation logic to automatically trust user-defined custom models.
• UI/UX Polishing: Integrated the model management controls into the existing filter bar with matching dimensions and typography for a cohesive look.

Test Plan

• Add a new custom model ID via the "Models" tab and verify it appears in the quota list.
• Delete a custom model and verify it is removed from the list and the configuration file.
• Go to "Settings" and map an existing model to a custom string; verify the change persists.
• Verify that autocomplete still works for pre-existing models in the mapping field.
• Run npm test to ensure no regressions in core proxy logic.