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/expenses/expense-form-dialog.tsx

@@ -393,7 +393,7 @@ export function ExpenseFormDialog({ open, onOpenChange, expense }: ExpenseFormDi
 
               {noReceipt && (
                 <div className="flex gap-2 rounded-md border border-amber-300 bg-amber-50 p-2 text-xs text-amber-900 dark:border-amber-900 dark:bg-amber-950/40 dark:text-amber-200">
-                  <AlertTriangle className="h-4 w-4 flex-shrink-0" />
+                  <AlertTriangle className="h-4 w-4 shrink-0" />
                   <span>
                     Expenses without a receipt may not be reimbursed by the parent company. The PDF
                     export will flag this expense.

src/components/expenses/trip-label-combobox.tsx

@@ -76,7 +76,7 @@ export function TripLabelCombobox({
           <ChevronsUpDown className="ml-2 h-4 w-4 shrink-0 opacity-50" />
         </Button>
       </PopoverTrigger>
-      <PopoverContent className="w-[var(--radix-popper-anchor-width)] min-w-[260px] p-0">
+      <PopoverContent className="w-(--radix-popper-anchor-width) min-w-[260px] p-0">
         <Command shouldFilter={true}>
           <CommandInput
             placeholder="Type a trip name…"