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/pages/UsersAdminPage.tsx

@@ -110,7 +110,11 @@ export function UsersAdminPage(): JSX.Element {
 
       <section className="card-product flex flex-col gap-md">
         <h2 className="text-[20px] font-medium">Create account</h2>
-        <form onSubmit={onCreate} className="grid grid-cols-1 md:grid-cols-4 gap-md items-end">
+        {/*
+          items-start so each cell top-aligns; the button is wrapped in a flex column
+          that pushes it to align with the input row (label + 4px gap = ~22px offset).
+        */}
+        <form onSubmit={onCreate} className="grid grid-cols-1 md:grid-cols-4 gap-md items-start">
           <FormField label="Username" htmlFor="new-username" required>
             <TextInput
               id="new-username"
@@ -137,7 +141,9 @@ export function UsersAdminPage(): JSX.Element {
               options={ROLE_OPTIONS}
             />
           </FormField>
-          <div>
+          {/* Button column: spacer matches label row height so input + button baselines align */}
+          <div className="flex flex-col gap-xs">
+            <div className="h-[22px]" aria-hidden="true" />
             <button type="submit" className="btn-primary w-full" disabled={createMutation.isPending}>
               {createMutation.isPending ? 'Creating…' : 'Create'}
             </button>