fix(frontend): post-design-review — slab token split + badge contrast + modal backdrop + dark shadows

- Add fixed slab/slab-text/slab-muted tokens so utility strip and footer never
  invert to near-white in dark mode (root token split: ink is themed text,
  slab is fixed dark surface)
- btn-ink uses fixed #111827 so confirm dialogs stay dark-on-dark readable
- Toast error surface switched to slab; success uses text-white (not text-ink-on)
- StatusBadge active and SimulationStatusBadge review_required/done use text-white
  instead of text-canvas/text-ink-on (prevents near-black text on colored pill
  in dark mode)
- Modal backdrops (MitreMatrixModal, ConfirmDialog) switched to .modal-backdrop
  class (fixed rgba(0,0,0,0.6)) instead of bg-ink/60 which turned near-white
- Card shadow lifted in dark mode via .dark .card-product override
- MitreMatrixModal panel uses shadow-floating-dark in dark mode
- UsersAdminPage form: items-start + explicit label-height spacer on button
  column for pixel-perfect baseline alignment (AC-17.3 structural fix)

92/92 tests passing, typecheck and lint clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ConfirmDialog.tsx

@@ -24,7 +24,7 @@ export function ConfirmDialog({
       aria-labelledby="confirm-dialog-title"
       className="fixed inset-0 z-50 flex items-center justify-center"
     >
-      <div className="absolute inset-0 bg-ink/40" onClick={onCancel} aria-hidden="true" />
+      <div className="modal-backdrop absolute inset-0" onClick={onCancel} aria-hidden="true" />
       <div className="relative card-product shadow-floating max-w-sm w-full mx-md flex flex-col gap-md">
         <h2 id="confirm-dialog-title" className="text-[20px] font-medium text-ink">
           {title}