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/src/middleware/validators.js

@@ -168,6 +168,16 @@ const updateProfileValidation = [
       return value.includes('tiktok.com');
     })
     .withMessage('Must be a valid TikTok URL (tiktok.com)'),
+  body('country')
+    .optional()
+    .trim()
+    .isLength({ max: 100 })
+    .withMessage('Country must be less than 100 characters'),
+  body('city')
+    .optional()
+    .trim()
+    .isLength({ max: 100 })
+    .withMessage('City must be less than 100 characters'),
   handleValidationErrors,
 ];