Fix preview server 500 error: use adapter-node output and preview environment

[Copilot]

Preview server failed with 500 errors because vite preview doesn't execute adapter-node builds, and production builds fetch from http://www.jungesmusical.de which is inaccessible locally.

Changes

Environment configuration:

• Added .env.preview with PUBLIC_HOMEPAGE_HOST=http://localhost:5001 for local preview builds

Build scripts:

• preview: Changed from vite preview to node dist/index.js to execute the actual adapter-node server
• build:preview: New script using vite build --mode preview
• build: Remains vite build for production (uses .env.production)

Error handling:

• Updated page loaders to properly propagate HTTP status codes instead of catching all errors as 500
• Added type guards to distinguish SvelteKit errors from network/parsing errors

Usage

Preview with local homepage on port 5001:

npm run build:preview
npm run preview

Production build:

npm run build

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• www.jungesmusical.de
  • Triggering command: /home/REDACTED/work/_temp/ghcca-node/node/bin/node node dist/index.js (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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