Fix settings update errors and render release notes compactly

[webup]

What

• Show a backend-specific message when the update check hits a browser fetch failure instead of surfacing the raw generic error.
• Tighten the settings update section layout to reduce vertical space and keep controls denser.
• Render GitHub release note bodies as Markdown so headings, lists, links, and inline code display properly.
• Add .codex/ to .gitignore so local Codex environment files are not tracked.

Why

The update section should explain backend connectivity failures clearly, keep the settings UI compact, and display release notes in readable formatted Markdown.

How

• Map generic browser fetch failures to a localized backend-unavailable message.
• Replace the raw release-note <pre> with a scoped Markdown renderer for headings, lists, inline code, emphasis, and links.
• Add focused unit coverage for fetch-failure messaging and release-note Markdown rendering.

Testing

• npx vitest run src/modules/settings/__tests__/UpdateSection.test.tsx
• npm run build
• Local in-app browser verification of the settings update section.