chore(deps-major): migrate tailwindcss to 4.3.1 2026-06-24

[Copilot]

This PR migrates the project from Tailwind CSS v3 patterns to Tailwind CSS 4.3.1, covering the breaking config and CSS entrypoint changes required for builds to keep working.
The update is intentionally scoped to Tailwind v4 migration surface only (dependency + CSS/PostCSS integration points).

• Dependency migration (v3 → v4.3.1)

  • Upgrades tailwindcss to 4.3.1.
  • Adds @tailwindcss/postcss and removes autoprefixer, aligning with Tailwind v4 PostCSS integration.

• PostCSS pipeline update

  • Replaces the old bare tailwindcss plugin entry with @tailwindcss/postcss in postcss.config.cjs.

• CSS entrypoint migration for Tailwind v4

  • Replaces v3 directives (@tailwind base/components/utilities) with v4 import syntax.
  • Explicitly loads the existing Tailwind config via @config since v4 no longer auto-loads config the same way.

/* src/app.css */
@config "../tailwind.config.cjs";
@import "tailwindcss";

Original prompt

Migrate the dependency 'tailwindcss' to 4.3.1 on a new branch and open ONE pull request titled 'chore(deps-major): migrate tailwindcss to 4.3.1 2026-06-24'. This is a KNOWN BREAKING major bump — work through the migration carefully using this assessment as the spec:

v3→4 is a ground-up rewrite and the project uses every removed v3 pattern: postcss.config.cjs has bare tailwindcss plugin (→ @tailwindcss/postcss), src/app.css uses @tailwind base/components/utilities (→ @import "tailwindcss"), tailwind.config.cjs is no longer auto-loaded (→ @config or migrate to CSS @theme), autoprefixer/postcss-import now built-in (remove). Behavioral default changes (border/ring colors). Build breaks until migrated. npx @tailwindcss/upgrade codemod automates most, or switch to the @tailwindcss/vite plugin. Small surface (1 CSS + 2 config files) but nothing renders until migrated.

Apply the code changes the breaking changes require, run the project's install + test/build, and make sure it is green before opening the PR. Keep the change scoped to THIS migration only (do not bump unrelated dependencies, do not refactor). Do NOT merge.