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

supabase-setup.sql

@@ -0,0 +1,53 @@
+-- CODE CRISPIES - Supabase Database Setup
+-- Run this in Supabase Dashboard → SQL Editor → New Query
+
+-- User progress table
+CREATE TABLE user_progress (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id UUID REFERENCES auth.users(id) ON DELETE CASCADE,
+  progress JSONB NOT NULL DEFAULT '{}',
+  user_code JSONB NOT NULL DEFAULT '{}',
+  settings JSONB NOT NULL DEFAULT '{}',
+  language TEXT DEFAULT 'en',
+  updated_at TIMESTAMPTZ DEFAULT NOW(),
+  created_at TIMESTAMPTZ DEFAULT NOW(),
+  UNIQUE(user_id)
+);
+
+-- Newsletter subscribers table
+CREATE TABLE newsletter_subscribers (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  email TEXT UNIQUE NOT NULL,
+  subscribed_at TIMESTAMPTZ DEFAULT NOW()
+);
+
+-- Row Level Security
+ALTER TABLE user_progress ENABLE ROW LEVEL SECURITY;
+ALTER TABLE newsletter_subscribers ENABLE ROW LEVEL SECURITY;
+
+-- Users can only access their own progress
+CREATE POLICY "Users can CRUD own progress"
+  ON user_progress FOR ALL
+  USING (auth.uid() = user_id)
+  WITH CHECK (auth.uid() = user_id);
+
+-- Anyone can subscribe to newsletter (public insert)
+CREATE POLICY "Anyone can subscribe to newsletter"
+  ON newsletter_subscribers FOR INSERT
+  WITH CHECK (true);
+
+-- Function to delete own account (called via RPC)
+CREATE OR REPLACE FUNCTION delete_own_account()
+RETURNS void
+LANGUAGE plpgsql
+SECURITY DEFINER
+SET search_path = public
+AS $$
+BEGIN
+  -- Delete user's progress (CASCADE should handle this, but be explicit)
+  DELETE FROM user_progress WHERE user_id = auth.uid();
+
+  -- Delete the user from auth.users
+  DELETE FROM auth.users WHERE id = auth.uid();
+END;
+$$;