feat: add country and city fields to user profile

- Add country and city fields to User model
- Create database migration for location fields
- Add validation for country and city (max 100 characters)
- Create countries.js with complete list of 195 countries
- Add country dropdown select and city text input to profile page
- Include country and city in GET /api/users/me response
- Update profile form to support location data

Users can now select their country from a dropdown list of all
countries and enter their city name.

backend/prisma/migrations/20251113194943_add_location_fields/migration.sql

@@ -0,0 +1,3 @@
+-- AlterTable
+ALTER TABLE "users" ADD COLUMN     "city" VARCHAR(100),
+ADD COLUMN     "country" VARCHAR(100);