docs(readme): showcase layout, local-first pitch, corrected formats

[Papyszoo]

Summary

Restructures the README around what first-time visitors need: a centered hero with tagline, dynamic badges, and demo call-to-action, followed by a "Why Modelibr" section that finally says local-first out loud.

Factual fixes

• Supported formats: added .stl / .3mf (shipped in 0.3.0 but missing from the README), plus previously undocumented format tables for textures, sprites, sounds, and scripts.
• Badges: replaced hand-maintained stack-version badges (the Three.js one had already drifted: claimed 0.180, app ships 0.185) with a dynamic release badge that can't go stale.
• License: states the actual BSL grant plainly — free to use/modify/self-host forever including commercially; the single restriction is running a competing public asset marketplace.

New content

• Scripts as a first-class asset type (feature bullet, tagline, formats row).
• Install section: installer filename patterns, host status window description, macOS unsigned-builds update caveat, Docker data-location/backup/update notes.
• Development section (collapsible): stack table, dev loop, test runner commands, contribution note (PRs target the version branch, conventional commits).

Note

The hero screenshot slot is a commented-out TODO (docs/static/img/screenshots/hero.png) — the existing E2E captures are too sparse to showcase. To be captured from a populated library and uncommented in a follow-up.