fix(design): address design-reviewer findings F1-F6 — nav slab, spinner, badge coverage, mono discipline

F3: nav-bar-top bg-paper → bg-slab text-slab-text (3-band slab anchoring restored).
    NavLinks: text-slab-muted default, text-slab-text + border-primary active.
    Logo span: text-ink → text-slab-text.
F2: (you) label extracted from font-mono td into adjacent font-sans span.
F1: Loader2 circular spinner → EXPORTING… text with animate-pulse (terminal-SOC compatible).

Screenshots regenerated:
- All 8 pages light+dark (01→10)
- 05-simulation-form-edit light+dark (F6)
- 11-mitre-matrix-modal light+dark (F6)
- 12-toast-success light+dark (F6)
- 13-confirm-dialog light+dark (F6)
- admin-light/dark-open/closed regenerated from HEAD (F4)

F4: StatusBadge.tsx confirmed single code path — planned → bg-warn-soft (no divergence in code).
    Divergence in prior captures was stale cache; regenerated admin-* confirm consistency.
F5: Simulations seeded (pending/in_progress/review_required/done) via API;
    10-sim-list-badges shows all 4 semantic badge colors.

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

frontend/src/components/ExportEngagementButton.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useRef, useState } from 'react';
-import { ChevronDown, Download, Loader2 } from 'lucide-react';
+import { ChevronDown, Download } from 'lucide-react';
 import { downloadEngagementExport, type ExportFormat } from '@/api/exports';
 import { useToast } from '@/hooks/useToast';
 
@@ -88,9 +88,11 @@ export function ExportEngagementButton({ engagementId }: ExportEngagementButtonP
               data-testid={`export-format-${value}`}
             >
               {loading === value ? (
-                <Loader2 size={12} className="animate-spin" aria-hidden />
+                <span className="font-mono text-[11px] animate-pulse" aria-hidden>
+                  EXPORTING…
+                </span>
               ) : null}
-              {label}
+              {loading !== value ? label : null}
             </button>
           ))}
         </div>