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/MitreMatrixModal.tsx

@@ -163,14 +163,14 @@ export function MitreMatrixModal({
 
   return (
     <div className="fixed inset-0 z-50 flex items-center justify-center">
-      <div className="absolute inset-0 bg-ink/60" onClick={onCancel} aria-hidden="true" />
+      <div className="modal-backdrop absolute inset-0" onClick={onCancel} aria-hidden="true" />
 
       <div
         ref={containerRef}
         role="dialog"
         aria-modal="true"
         aria-labelledby="matrix-modal-title"
-        className="relative bg-canvas rounded-xl shadow-floating max-w-[98vw] max-h-[80vh] overflow-hidden flex flex-col"
+        className="relative bg-canvas rounded-xl shadow-floating dark:shadow-floating-dark max-w-[98vw] max-h-[80vh] overflow-hidden flex flex-col"
         style={{ width: '1400px' }}
         onKeyDown={handleKeyDown}
       >