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/shared/timezone-combobox.tsx

@@ -97,10 +97,7 @@ export function TimezoneCombobox({
           <ChevronsUpDown className="ml-2 h-4 w-4 shrink-0 opacity-50" />
         </Button>
       </PopoverTrigger>
-      <PopoverContent
-        className="w-[var(--radix-popper-anchor-width)] min-w-[360px] p-0"
-        align="start"
-      >
+      <PopoverContent className="w-(--radix-popper-anchor-width) min-w-[360px] p-0" align="start">
         <Command>
           <CommandInput placeholder="Search timezones…" />
           <CommandList>