Respect optional settings.json in shared container

[cyanzhong]

No description provided.

[Copilot]

🟡 Changes recommended

HTML-sensitive settings strings can break inline-script parsing and prevent Quick Look initialization.

1 open issue

• Escape < in inline-script JSON to prevent HTML parser breakage · New

What changed in this PR

Allows shared-container consumers, including Quick Look, to load optional Shared/settings.json.

Changes:

• Reads and validates settings, falling back to an empty object when unavailable or invalid.
• Adds tests for missing, invalid, unreadable, updated, and string-valued settings.

File	Description
MarkEditCore/​Tests/​EditorIndexHtmlTests.swift	Tests settings loading and value preservation.
MarkEditCore/​Sources/​EditorIndexHtml.swift	Loads shared settings and logs failures.

_{Review effort: Balanced}