Support specifying settings.json to configure the HTML Viewer

[willeastcott]

• Remove --camera-pos and --camera-target CLI options.
• Add --viewer-settings option that exposes the documented viewer settings schema.

• I confirm I have read the contributing guidelines

[Copilot]

Pull Request Overview

This PR replaces camera-specific CLI options with a flexible settings file approach for the HTML viewer, allowing users to configure all viewer settings through a JSON file rather than just camera position and target.

• Removed --camera-pos and --camera-target CLI options
• Added --viewer-settings option that accepts a JSON settings file
• Updated documentation and help text to reflect the new configuration approach

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/writers/write-html.ts	Implements JSON settings file loading and merging with default settings
src/index.ts	Updates CLI argument parsing to use new viewer-settings option
README.md	Updates documentation to reflect new settings file approach

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