feat: add authentication, cloud sync, and GDPR compliance

Authentication & Cloud Sync:
- Add Supabase integration for auth (email/password, Google, GitHub OAuth)
- Add cloud progress sync for logged-in users
- Add account deletion feature with confirmation dialog
- Auth is optional - anonymous users can still use localStorage

UI Improvements:
- Add dark-themed account section in sidebar
- Show user email in header when logged in
- Add signup success feedback message
- Update landing page: remove cloud sync from Coming Soon, add Code Challenges
- Update benefit text to mention optional cloud sync

GDPR Compliance:
- Add Privacy Policy dialog with full GDPR-compliant content
- Add Imprint dialog with legal contact information
- Add footer links for Privacy and Imprint
- All legal content translated to 6 languages (en, de, pl, es, ar, uk)

Files added:
- src/supabase.js - Supabase client with auth and progress sync helpers
- src/auth.js - Authentication logic and form handlers
- supabase-setup.sql - Database schema and RLS policies

.gitignore

@@ -3,6 +3,8 @@
 node_modules
 dist
 coverage
+.env
+.env.local
 
 # Claude Code local settings (user-specific)
 .claude/settings.local.json