feat(web): add Modal + EmptyState primitives and a11y hooks

- Modal: shared backdrop/panel with ESC-close, backdrop-click-close,
  focus trap, body scroll lock; supports center + drawer-right variants,
  matrix/violet accents, default/wide widths.
- EmptyState: icon + title + hint + optional CTA; compact variant
  for tight rails.
- useEscapeKey, useFocusTrap: reusable hooks powering Modal; will also
  be adopted by CommandPalette and ContextMenu in follow-up commits.

No retrofits yet — primitives only. tsc clean.

decnet_web/src/components/EmptyState/EmptyState.css

@@ -0,0 +1,52 @@
+/* Shared EmptyState styling. Component-scoped .empty-state rules
+   (Attackers/Bounty/LiveLogs) still win over these base rules. */
+
+.empty-state-icon {
+  opacity: 0.4;
+  color: var(--violet);
+}
+
+.empty-state-title {
+  font-size: 0.75rem;
+  letter-spacing: 2px;
+  opacity: 0.7;
+}
+
+.empty-state-hint {
+  font-size: 0.65rem;
+  opacity: 0.45;
+  letter-spacing: 1px;
+  margin-top: -4px;
+}
+
+.empty-state-cta {
+  margin-top: 6px;
+  padding: 6px 12px;
+  background: transparent;
+  border: 1px solid var(--matrix);
+  color: var(--matrix);
+  font-family: var(--font-mono);
+  font-size: 0.65rem;
+  letter-spacing: 2px;
+  cursor: pointer;
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  text-transform: uppercase;
+}
+
+.empty-state-cta:hover {
+  background: rgba(0, 255, 65, 0.1);
+  box-shadow: 0 0 8px rgba(0, 255, 65, 0.3);
+}
+
+.empty-state.empty-state-compact {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  padding: 6px 4px;
+  font-size: 0.65rem;
+  letter-spacing: 1.5px;
+  opacity: 0.5;
+  color: var(--text-dim, #888);
+}

decnet_web/src/components/EmptyState/EmptyState.tsx

@@ -0,0 +1,46 @@
+import React from 'react';
+import type { LucideIcon } from 'lucide-react';
+import './EmptyState.css';
+
+interface CTA {
+  label: string;
+  onClick: () => void;
+  icon?: LucideIcon;
+}
+
+interface Props {
+  icon?: LucideIcon;
+  title: string;
+  hint?: string;
+  cta?: CTA;
+  size?: 'default' | 'compact';
+  className?: string;
+}
+
+const EmptyState: React.FC<Props> = ({ icon: Icon, title, hint, cta, size = 'default', className = '' }) => {
+  if (size === 'compact') {
+    return (
+      <div className={`empty-state empty-state-compact ${className}`}>
+        {Icon && <Icon size={12} />}
+        <span>{title}</span>
+      </div>
+    );
+  }
+
+  const CtaIcon = cta?.icon;
+  return (
+    <div className={`empty-state ${className}`}>
+      {Icon && <Icon size={28} className="empty-state-icon" />}
+      <div className="type-label empty-state-title">{title}</div>
+      {hint && <div className="empty-state-hint">{hint}</div>}
+      {cta && (
+        <button type="button" className="empty-state-cta" onClick={cta.onClick}>
+          {CtaIcon && <CtaIcon size={12} />}
+          <span>{cta.label}</span>
+        </button>
+      )}
+    </div>
+  );
+};
+
+export default EmptyState;