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/Modal/Modal.css

@@ -0,0 +1,24 @@
+/* Drawer-right variant: reuses .modal-backdrop + .modal base from DeckyFleet.css
+   and shifts the panel to the right edge as a full-height side panel. */
+
+.modal-backdrop.drawer {
+  justify-content: flex-end;
+  align-items: stretch;
+}
+
+.modal.modal-drawer-right {
+  width: 520px;
+  max-width: 96vw;
+  max-height: 100vh;
+  height: 100vh;
+  border-left: 1px solid var(--matrix);
+  border-top: none;
+  border-right: none;
+  border-bottom: none;
+  box-shadow: -8px 0 30px rgba(0, 0, 0, 0.6);
+}
+
+.modal.modal-drawer-right.violet {
+  border-left-color: var(--violet);
+  box-shadow: -8px 0 30px rgba(238, 130, 238, 0.25);
+}

decnet_web/src/components/Modal/Modal.tsx

@@ -0,0 +1,85 @@
+import React, { useEffect, useRef } from 'react';
+import { X, type LucideIcon } from 'lucide-react';
+import { useEscapeKey } from '../../hooks/useEscapeKey';
+import { useFocusTrap } from '../../hooks/useFocusTrap';
+import './Modal.css';
+
+interface Props {
+  open: boolean;
+  onClose: () => void;
+  title?: string;
+  icon?: LucideIcon;
+  footer?: React.ReactNode;
+  accent?: 'matrix' | 'violet';
+  width?: 'default' | 'wide';
+  variant?: 'center' | 'drawer-right';
+  children: React.ReactNode;
+  className?: string;
+}
+
+const Modal: React.FC<Props> = ({
+  open,
+  onClose,
+  title,
+  icon: Icon,
+  footer,
+  accent = 'matrix',
+  width = 'default',
+  variant = 'center',
+  children,
+  className = '',
+}) => {
+  const panelRef = useRef<HTMLDivElement | null>(null);
+
+  useEscapeKey(onClose, open);
+  useFocusTrap(panelRef, open);
+
+  useEffect(() => {
+    if (!open) return;
+    const prev = document.body.style.overflow;
+    document.body.style.overflow = 'hidden';
+    return () => {
+      document.body.style.overflow = prev;
+    };
+  }, [open]);
+
+  if (!open) return null;
+
+  const panelClasses = [
+    'modal',
+    accent === 'violet' ? 'violet' : '',
+    width === 'wide' ? 'wide' : '',
+    variant === 'drawer-right' ? 'modal-drawer-right' : '',
+    className,
+  ].filter(Boolean).join(' ');
+
+  const backdropClass = variant === 'drawer-right' ? 'modal-backdrop drawer' : 'modal-backdrop';
+
+  return (
+    <div className={backdropClass} onClick={onClose}>
+      <div
+        ref={panelRef}
+        className={panelClasses}
+        onClick={(e) => e.stopPropagation()}
+        role="dialog"
+        aria-modal="true"
+      >
+        {title && (
+          <div className="modal-head">
+            <h3>
+              {Icon && <Icon size={14} />}
+              {title}
+            </h3>
+            <button className="close-btn" onClick={onClose} aria-label="Close">
+              <X size={16} />
+            </button>
+          </div>
+        )}
+        {children}
+        {footer && <div className="modal-foot">{footer}</div>}
+      </div>
+    </div>
+  );
+};
+
+export default Modal;