Add optional KaTeX math rendering toggle (usekatex / usemath) for web UI

[nmaguiar]

Motivation

• Provide an optional, non-breaking way to render LaTeX math in the browser UI similar to the existing Mermaid support so users can opt-in to math rendering when desired.
• Avoid changing default behavior or core Mini-A arguments while exposing the feature to the web UI and server endpoints.

Description

• Add usekatex and alias usemath parameters to mini-a-web.yaml, expose usekatex in the /info response as usekatex, and normalize the usemath alias to usekatex before use in the server startup logic.
• Keep the KaTeX toggle out of core Mini-A runtime args by deleting usekatex from global.maArgs so it only impacts the browser UI and server state.
• Wire the browser markdown pipeline in public/index.md to optionally load KaTeX assets and the showdown-katex extension, dynamically build a showdown.Converter with the extension when enabled, and add functions ensureKatexAssets / configureKatexAvailability to manage asset loading.
• Integrate feature detection into the UI flow by reading usekatex from /info inside configureFeatureAvailability() and calling configureKatexAvailability(); document the new flag in USAGE.md.

Testing

• No automated tests were executed for this change.