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

src/impl/LessonEngine.js

@@ -4,6 +4,20 @@
  */
 import { validateUserCode } from "../helpers/validator.js";
 
+// Auth sync - lazy loaded to avoid circular dependencies
+let authModule = null;
+async function getAuthModule() {
+	if (!authModule) {
+		try {
+			authModule = await import("../auth.js");
+		} catch (e) {
+			// Auth module not available, skip cloud sync
+			return null;
+		}
+	}
+	return authModule;
+}
+
 export class LessonEngine {
 	constructor() {
 		this.currentLesson = null;
@@ -484,7 +498,7 @@ export class LessonEngine {
 	}
 
 	/**
-	 * Save progress to localStorage
+	 * Save progress to localStorage and optionally sync to cloud
 	 */
 	saveUserProgress() {
 		try {
@@ -494,11 +508,24 @@ export class LessonEngine {
 				timestamp: new Date().toISOString()
 			};
 			localStorage.setItem("codeCrispies.progress", JSON.stringify(progressData));
+
+			// Trigger cloud sync if logged in (debounced)
+			this.triggerCloudSync();
 		} catch (e) {
 			console.error("Error saving progress:", e);
 		}
 	}
 
+	/**
+	 * Trigger cloud sync if user is logged in (debounced)
+	 */
+	async triggerCloudSync() {
+		const auth = await getAuthModule();
+		if (auth?.isLoggedIn()) {
+			auth.debouncedSyncToCloud();
+		}
+	}
+
 	/**
 	 * Load progress from localStorage
 	 */
@@ -521,11 +548,14 @@ export class LessonEngine {
 	}
 
 	/**
-	 * Save user code to localStorage
+	 * Save user code to localStorage and optionally sync to cloud
 	 */
 	saveUserCodeToStorage() {
 		try {
 			localStorage.setItem("codeCrispies.userCode", JSON.stringify(Array.from(this.userCodeMap.entries())));
+
+			// Trigger cloud sync if logged in (debounced)
+			this.triggerCloudSync();
 		} catch (e) {
 			console.error("Error saving user code:", e);
 		}