feat(package): stabilize CSS export

[alx-xo]

Summary

• Update the public ./moonshine.css export to point at the built CSS artifact, dist/moonshine.css
• Pin Vite's library CSS filename to moonshine so the export map no longer depends on an implicit tool default
• Mark CSS files as package side effects so consumer bundlers preserve Moonshine styles during tree-shaking

Root Cause

• The Vite 8 upgrade changed the library CSS build artifact from dist/style.css to dist/moonshine.css.
• Moonshine's package export map still resolved @speakeasy-api/moonshine/moonshine.css to dist/style.css, so consumers could import the documented public path but their bundler could not find the target file.
• The package also declared sideEffects: false despite importing CSS, which made the CSS contract brittle for consumer bundlers that tree-shake side-effect imports.

Test Plan

• pnpm test --run
• pnpm type-check
• pnpm lint
• pnpm build
• node export metadata check confirmed ./moonshine.css resolves to an existing dist/moonshine.css file and CSS sideEffects are declared