Delete tailwind.config.ts and migrate to CSS-native declarations

[kmcginnes]

Description

Eliminates the JS config file entirely by migrating all its remaining contents to Tailwind v4 CSS-native declarations. After this, index.css is the single source of truth for all styling tokens, utilities, and theme values — no JS config, no @config directive.

What moved:

• Logo gradient, max-width, keyframes/animations → @theme block
• content-auto/hidden/visible → @utility declarations
• intrinsic-size-* → functional @utility with --spacing(--value(integer))
• Semantic z-index scale → @utility declarations (z-index has no theme namespace in v4)
• data-open/data-closed → @custom-variant (Radix's data-state convention)
• transition-width → transition-[width] (native arbitrary value syntax)

What was dropped (native in v4):

• extend.aria.invalid — built-in aria-invalid: variant
• extend.data.open/closed — replaced by @custom-variant above
• fontFamily — Tailwind default sans renders same system font (Nunito Sans was never loaded)

Verified working: production build compiles all custom utilities into output CSS, all 2178 tests pass.

How to read

1. tailwind.config.ts (deleted) — everything here moved somewhere else
2. index.css — the new home: @custom-variant, @theme, and @utility blocks
3. CollapsibleSidebar.tsx — transition-width → transition-[width]
4. vite.config.ts — remove stale coverage exclusion
5. design.md — update migration banner and source-of-truth section

Validation

• pnpm checks passes
• pnpm test passes (2178 tests)
• pnpm build passes — verified custom utilities (content-visibility, contain-intrinsic-size, z-index, logo-gradient, animations) all appear in compiled CSS output

Related Issues

• Closes Delete tailwind.config.ts and migrate to CSS-native declarations #1954
• Part of Define design system and align codebase styling #1952

Check List

• I confirm that my contribution is made under the terms of the Apache 2.0 license.
• I have verified pnpm checks passes with no errors.
• I have verified pnpm test passes with no failures.
• I have covered new added functionality with unit tests if necessary.
• I have updated documentation if necessary.