feat: Remove localStorage for game states and implement cross-domain player ID sync

- Remove localStorage for game states and statistics (backend only)
- Add API route to suggest player ID based on recently updated states
- Add async player ID lookup that finds existing IDs across domains
- When visiting a new domain, automatically find and use existing player ID
- Enables cross-domain synchronization between hoerdle.de and hördle.de

app/api/player-id/suggest/route.ts

@@ -0,0 +1,67 @@
+import { NextResponse } from 'next/server';
+import { PrismaClient } from '@prisma/client';
+
+const prisma = new PrismaClient();
+
+/**
+ * POST /api/player-id/suggest
+ * 
+ * Tries to find a player ID based on recently updated states for a genre.
+ * This helps synchronize player IDs across different domains (hoerdle.de and hördle.de).
+ * 
+ * Request body:
+ * - genreKey: Genre key (e.g., "global", "Rock", "special:00725")
+ * 
+ * Returns:
+ * - playerId: Suggested player ID (UUID) if found, null otherwise
+ */
+export async function POST(request: Request) {
+    try {
+        const body = await request.json();
+        const { genreKey } = body;
+
+        if (!genreKey || typeof genreKey !== 'string') {
+            return NextResponse.json(
+                { error: 'Missing or invalid genreKey' },
+                { status: 400 }
+            );
+        }
+
+        // Find the most recently updated player state for this genre
+        // Look for states updated in the last 48 hours
+        const cutoffDate = new Date();
+        cutoffDate.setHours(cutoffDate.getHours() - 48);
+
+        const recentState = await prisma.playerState.findFirst({
+            where: {
+                genreKey: genreKey,
+                lastPlayed: {
+                    gte: cutoffDate,
+                },
+            },
+            orderBy: {
+                lastPlayed: 'desc',
+            },
+        });
+
+        if (recentState) {
+            // Return the player ID from the most recent state
+            return NextResponse.json({
+                playerId: recentState.identifier,
+                lastPlayed: recentState.lastPlayed,
+            });
+        }
+
+        // No recent state found
+        return NextResponse.json({
+            playerId: null,
+        });
+    } catch (error) {
+        console.error('[player-id/suggest] Error finding player ID:', error);
+        return NextResponse.json(
+            { error: 'Internal Server Error' },
+            { status: 500 }
+        );
+    }
+}
+