style: support and document Tailwind CSS 3 and 4 installation paths

[codeacme17]

What to build

Adopt Tailwind CSS 4 as the recommended Echo UI styling path while giving Tailwind CSS 3 projects an honest, tested installation path. The bilingual installation guide must show both configurations and name the Echo UI version range that each path actually supports.

Acceptance criteria

• The library styling pipeline, class composition utilities, theme tokens, and example application are migrated to the selected Tailwind 4 integration.
• Separate Tailwind 3 and Tailwind 4 consumer fixtures build representative Echo UI components from the published package shape.
• The English and Chinese installation guides show the required Tailwind 4 imports/source discovery and the Tailwind 3 content/theme configuration.
• A compatibility table maps Echo UI, Tailwind CSS, tailwind-variants, and tailwind-merge versions.
• The current Echo UI release is advertised as Tailwind 3 compatible only if its Tailwind 3 consumer fixture passes; otherwise the guide pins the last compatible Echo UI release.
• Tailwind 3 to 4 migration guidance and visual regression coverage address changed utility semantics.

Blocked by

• build: upgrade the library build and test stack to Vite 8 and Vitest 4 #78 — build: upgrade the library build and test stack to Vite 8 and Vitest 4.
• docs: cut over to Nextra and remove IslandJS #84 — docs: cut over to Nextra and remove IslandJS.