feat(deps): Tailwind 3 → 4 + swap tailwindcss-animate for tw-animate-css

Ran the official @tailwindcss/upgrade tool:
- tailwind.config.ts → @theme directive in globals.css
- @tailwind base/components/utilities → @import 'tailwindcss'
- postcss.config switched from tailwindcss + autoprefixer to
  @tailwindcss/postcss (autoprefixer baked in)
- focus-visible:outline-none → focus-visible:outline-hidden (the v3
  utility was a footgun — outline still showed in forced-colors mode)

Reverted the migration tool's over-zealous variant="outline" →
variant="outline-solid" rename on CVA prop values; that rename was
meant for the Tailwind `outline:` utility, not our Button/Badge
component variants.

Swapped tailwindcss-animate (v3-style JS plugin) for tw-animate-css
(v4-native @import). Same utility surface (animate-spin, animate-in,
etc.), one fewer JS plugin in the bundle.

Fixed the upgrade tool's malformed dark variant
(@custom-variant dark (&:is(class *)) — `class` was being parsed as
a tag) to canonical &:where(.dark, .dark *).

Verified: tsc 0 errors, eslint 0 errors (16 pre-existing warnings),
vitest 1315/1315, next build clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/ui/button.tsx

@@ -5,7 +5,7 @@ import { cva, type VariantProps } from 'class-variance-authority';
 import { cn } from '@/lib/utils';
 
 const buttonVariants = cva(
-  'inline-flex items-center justify-center gap-2 whitespace-nowrap rounded-md text-sm font-medium transition-colors focus-visible:outline-none focus-visible:ring-1 focus-visible:ring-ring disabled:pointer-events-none disabled:opacity-50 [&_svg]:pointer-events-none [&_svg]:size-4 [&_svg]:shrink-0',
+  'inline-flex items-center justify-center gap-2 whitespace-nowrap rounded-md text-sm font-medium transition-colors focus-visible:outline-hidden focus-visible:ring-1 focus-visible:ring-ring disabled:pointer-events-none disabled:opacity-50 [&_svg]:pointer-events-none [&_svg]:size-4 [&_svg]:shrink-0',
   {
     variants: {
       variant: {