Add Country id route and update routes to use countryId path param

[SakshiKekre]

Fixes #196

Implemented country-based URL routing to support multi-country functionality:

• Modified Router structure to include /:countryId prefix for all routes
• Added useCurrentCountry() hook to get country ID from URL params
• Updated Sidebar navigation to dynamically include country prefix in all internal paths
• All routes now follow pattern: /:countryId/[page] (e.g., /us/reports, /uk/simulations)

[EDIT] Updates since last review

Fixed country routing validation issue

Issue/Bug: Invalid paths like /configurations were incorrectly accepted as country codes and router would not redirect/navigate to country path.

Changes made:

1. Added loader-based validation - Created validateCountryLoader that validates country parameters at the router level before components render
2. Removed duplicate validation - Simplified useCurrentCountry hook to trust the loader's validation, removing redundant checks
3. Proper redirect handling - Invalid countries now redirect to default country (US) while preserving the rest of the URL path
4. Test coverage - Added tests for the new loader with proper fixtures following project conventions

Technical details:

• Router now uses a loader to validate :countryId parameter against valid country list
• Invalid routes like /configurations redirect to /us
• Routes with invalid countries like /invalid/policies redirect to /us/policies
• Hook no longer validates (Single Responsibility Principle) - it trusts the loader did its job
• All tests updated to reflect new validation flow

[anth-volk]

Thanks for this @SakshiKekre. I have a few questions in here and mostly nits.

[SakshiKekre]

@anth-volk Thanks for the review. I have made changes based on your comments + bug fix (please see PR description for more).

[anth-volk]

Thanks for this @SakshiKekre. My main takeaway here: I really don't think we should be using loaders if we can avoid it. I think the code as it is is probably fine, but I wouldn't be surprised if down the road, we added something and literally rendered a white screen for a perceptible amount of time.

[anth-volk]

Let's move with this. If we find ourselves adding too many components that don't fit inside the layout component we have, we may want to modify.