Docs: recovery screen for a damaged config file

[SandObserver]

Summary

• Troubleshooting: add "Stackyard cannot read its settings file" for a damaged or unreadable config, covering the two causes, the backup copy, the steps to fix it, the Check again button and the log line. The app links to this section by its anchor.
• Troubleshooting: drop the "starts empty" explanation from "My dashboard is empty after a restart", and add the recovery screen title to Messages you might see.
• Backup and restore: a corrupt config now keeps sign-in and saving off instead of starting empty, and restoring an export needs the password set again.

Testing
npm run build and npm run check pass. Text only; the screen title and log line are copied from the app's ui/i18n/en.json and api/src/config.js.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

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