docs: clarify the README and getting started

[MrScripty]

Summary

• Lead with sharing one local model library across applications.
• Put supported desktop and headless source routes first, including their native build prerequisites.
• Link maintained API and operational guidance while preserving ownership, runtime and platform limits.
• Retain the existing branding banner; this change does not add a UI screenshot.

Verification

• Revalidated current main and checked command entry points and relative links against source.
• Independently reviewed the final README and visually inspected the retained banner.
• Runtime commands were not executed for this documentation-only edit.

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