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/Toast.tsx

@@ -15,10 +15,11 @@ export function ToastViewport(): JSX.Element {
       {toasts.map((t) => {
         const isError = t.kind === 'error';
         const isSuccess = t.kind === 'success';
+        // Fixed colors: toasts don't theme (error=dark slab, success=primary blue)
         const surface = isError
-          ? 'bg-ink text-ink-on'
+          ? 'bg-slab text-slab-text'
           : isSuccess
-            ? 'bg-primary text-ink-on'
+            ? 'bg-primary text-white'
             : 'bg-canvas text-ink border border-hairline';
         return (
           <div