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/layout/mobile/mobile-topbar.tsx

@@ -38,7 +38,7 @@ export function MobileTopbar() {
     <header
       className={cn(
         'fixed top-0 inset-x-0 z-40',
-        'bg-gradient-to-b from-[#1e2844] to-[#171f35]',
+        'bg-linear-to-b from-[#1e2844] to-[#171f35]',
         'shadow-[0_4px_18px_-6px_rgba(15,23,42,0.45)]',
         'h-[calc(56px+env(safe-area-inset-top))] pt-safe-top',
         'flex items-center gap-2 px-3',

src/components/layout/sidebar.tsx

@@ -376,7 +376,7 @@ function SidebarContent({
                 <button
                   type="button"
                   aria-label="Open user menu"
-                  className="rounded-full focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-[#3a7bc8] focus-visible:ring-offset-2 focus-visible:ring-offset-white"
+                  className="rounded-full focus-visible:outline-hidden focus-visible:ring-2 focus-visible:ring-[#3a7bc8] focus-visible:ring-offset-2 focus-visible:ring-offset-white"
                 >
                   <Avatar className="w-8 h-8 cursor-pointer">
                     <AvatarImage src={undefined} />
@@ -396,7 +396,7 @@ function SidebarContent({
                 <button
                   type="button"
                   aria-label="Open user menu"
-                  className="flex w-full items-center gap-3 rounded-md p-1.5 text-left transition-colors hover:bg-accent focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-[#3a7bc8] focus-visible:ring-offset-2 focus-visible:ring-offset-white"
+                  className="flex w-full items-center gap-3 rounded-md p-1.5 text-left transition-colors hover:bg-accent focus-visible:outline-hidden focus-visible:ring-2 focus-visible:ring-[#3a7bc8] focus-visible:ring-offset-2 focus-visible:ring-offset-white"
                 >
                   <Avatar className="w-8 h-8 shrink-0 shadow-sm ring-2 ring-slate-200">
                     <AvatarImage src={undefined} />