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/tailwind.config.ts

@@ -34,6 +34,10 @@ const config: Config = {
         },
         charcoal: 'var(--color-charcoal)',
         graphite: 'var(--color-graphite)',
+        // Fixed dark slab — never inverts in dark mode (utility strip, footer, dark bands)
+        slab: '#111827',
+        'slab-text': '#f9fafb',
+        'slab-muted': '#6b7280',
         // Semantic / decorative — fixed (not themeable)
         bloom: {
           coral: '#ff5050',
@@ -91,6 +95,8 @@ const config: Config = {
       boxShadow: {
         'soft-lift': '0 2px 8px rgba(26, 26, 26, 0.08)',
         floating: '0 8px 24px rgba(26, 26, 26, 0.12)',
+        'soft-lift-dark': '0 2px 8px rgba(0, 0, 0, 0.32)',
+        'floating-dark': '0 8px 24px rgba(0, 0, 0, 0.48)',
       },
       maxWidth: {
         page: '1366px',