Startup hardening: safe mode, a software-renderer retry, and bad settings files set aside (#1138)

Startup no longer stops at a bad settings file or a missing GPU: a settings file that fails to load is set aside, a failed GPU start retries with the software renderer, and a safe mode starts with defaults. Adds a troubleshooting page to the guide.

Cargo.lock

@@ -3457,8 +3457,10 @@ dependencies = [
  "objc2 0.6.5",
  "objc2-app-kit 0.3.2",
  "objc2-foundation 0.3.2",
+ "pollster 0.4.0",
  "quick-xml",
  "rfd",
+ "ron",
  "rustls",
  "serde_json",
  "solidcraft-engine",