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.

docs/book/src/SUMMARY.md

@@ -15,6 +15,7 @@
 - [Import and export](guide/files.md)
 - [Keyboard shortcuts](guide/shortcuts.md)
 - [Updates and privacy](guide/updates.md)
+- [Troubleshooting: SolidCraft won't start](guide/troubleshooting.md)
 - [Driving SolidCraft with an AI agent (MCP)](guide/mcp.md)
 
 # Developer guide