Revert tailwindcss v4 bump (#62) — restore production map rendering

[nx-thaddeusaid]

Summary

Reverts the tailwindcss 3.4.19 → 4.3.0 upgrade merged in #62. Production at https://www.roguewar.org/warmap is currently rendering as a black page because Tailwind utilities are no longer being emitted into the bundle, which collapses the Konva <Stage> container to 0×0.

Why this is a hotfix, not a forward migration

Tailwind v4 is a major release with breaking syntax that the codebase still uses in v3 form:

• src/index.css uses @tailwind base; @tailwind components; @tailwind utilities; — v4 requires @import "tailwindcss";.
• postcss.config.js uses tailwindcss: {} as the PostCSS plugin — v4 split that into a separate @tailwindcss/postcss package.
• @material-tailwind/react@^2.1.10 is a v3-era component lib and is unlikely to work against v4 without changes.

With v4 installed against v3 syntax, PostCSS silently produces a stylesheet with no Tailwind utilities. CI (.github/workflows/test.yml) runs lint + typecheck + vitest only — no CSS-output check — so the regression shipped silently.

A proper v4 migration should be a separate, audited PR (CSS imports, postcss plugin swap, @material-tailwind/react compatibility check, visual regression).

What this PR changes

Single revert commit. Only package.json and yarn.lock change — tailwindcss returns to ^3.4.12.

Test plan

• After merge, confirm https://www.roguewar.org/warmap renders the galaxy map (not a black page).
• yarn install && yarn build succeeds locally.
• yarn test passes.