Add "Load from URL" tab to GTFS selector

[chtitux]

Summary

This PR adds a new "Load from URL" feature to the GTFS selector, allowing users to input a GTFS feed URL directly instead of only being able to upload files or search predefined sources.

Key Changes

• New UrlInput component: A reusable form component that accepts a GTFS feed URL, trims whitespace, and calls a callback with the result
• "Load from URL" tab: Added to GtfsSelector as a new tab option alongside file import and source search
• Styling: Added comprehensive CSS styles for the URL input form with focus states and hover effects
• Exports: Exported UrlInput and UrlInputProps from the main index for standalone use
• Tests: Added comprehensive test suite covering rendering, form submission, empty input validation, whitespace trimming, and className prop

Implementation Details

• The UrlInput component uses React hooks (useState, useCallback) for state management
• Form submission is prevented with e.preventDefault() and validates that the URL is not empty after trimming
• The component integrates seamlessly with the existing tab-based UI pattern in GtfsSelector
• Accessibility features included: proper ARIA labels, semantic HTML form elements, and keyboard support
• Styling follows the existing design system with consistent spacing, colors, and transitions

https://claude.ai/code/session_01C57bNoi7igqU1xEtLS2Coy