feat: add social media links to user profile

- Add YouTube, Instagram, Facebook, and TikTok URL fields to User model
- Create database migration for social media link columns
- Add custom validators to ensure URLs contain correct domains
- Update profile page with social media input fields
- Include social media URLs in GET /api/users/me response
- Add icons for each social platform in the UI

Users can now add links to their social media profiles. Each field
validates that the URL contains the appropriate domain (e.g.,
instagram.com for Instagram, youtube.com/youtu.be for YouTube).

backend/prisma/schema.prisma

@@ -23,6 +23,12 @@ model User {
   lastName                String?   @map("last_name") @db.VarChar(100)
   wsdcId                  String?   @unique @map("wsdc_id") @db.VarChar(20)
 
+  // Social Media Links
+  youtubeUrl              String?   @map("youtube_url") @db.VarChar(255)
+  instagramUrl            String?   @map("instagram_url") @db.VarChar(255)
+  facebookUrl             String?   @map("facebook_url") @db.VarChar(255)
+  tiktokUrl               String?   @map("tiktok_url") @db.VarChar(255)
+
   // Email Verification (Phase 1.5)
   emailVerified           Boolean   @default(false) @map("email_verified")
   verificationToken       String?   @unique @map("verification_token") @db.VarChar(255)