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/schema.prisma

@@ -29,6 +29,10 @@ model User {
   facebookUrl             String?   @map("facebook_url") @db.VarChar(255)
   tiktokUrl               String?   @map("tiktok_url") @db.VarChar(255)
 
+  // Location
+  country                 String?   @db.VarChar(100)
+  city                    String?   @db.VarChar(100)
+
   // Email Verification (Phase 1.5)
   emailVerified           Boolean   @default(false) @map("email_verified")
   verificationToken       String?   @unique @map("verification_token") @db.VarChar(255)