fix: clean Next.js build artifacts in setup script

Add cleanup step to remove packages/web/.next directory before building.
This prevents "Cannot find module" errors from stale webpack artifacts
that can occur after rebasing or switching branches.

The cleanup step ensures a clean build every time the setup script runs.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

scripts/setup.sh

@@ -16,6 +16,9 @@ fi
 echo "📦 Installing dependencies..."
 pnpm install
 
+echo "🧹 Cleaning stale build artifacts..."
+rm -rf packages/web/.next
+
 echo "🔨 Building all packages..."
 pnpm build