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/hooks/useEscapeKey.ts

@@ -0,0 +1,15 @@
+import { useEffect } from 'react';
+
+export function useEscapeKey(onEscape: () => void, active: boolean = true): void {
+  useEffect(() => {
+    if (!active) return;
+    const handler = (e: KeyboardEvent) => {
+      if (e.key === 'Escape') {
+        e.stopPropagation();
+        onEscape();
+      }
+    };
+    window.addEventListener('keydown', handler);
+    return () => window.removeEventListener('keydown', handler);
+  }, [onEscape, active]);
+}

decnet_web/src/hooks/useFocusTrap.ts

@@ -0,0 +1,49 @@
+import { useEffect, type RefObject } from 'react';
+
+const FOCUSABLE =
+  'input:not([disabled]), button:not([disabled]), textarea:not([disabled]), select:not([disabled]), a[href], [tabindex]:not([tabindex="-1"])';
+
+export function useFocusTrap(
+  ref: RefObject<HTMLElement | null>,
+  active: boolean,
+): void {
+  useEffect(() => {
+    if (!active || !ref.current) return;
+    const root = ref.current;
+    const previouslyFocused = document.activeElement as HTMLElement | null;
+
+    const focusables = () =>
+      Array.from(root.querySelectorAll<HTMLElement>(FOCUSABLE)).filter(
+        (el) => !el.hasAttribute('aria-hidden') && el.offsetParent !== null,
+      );
+
+    const autoFocus =
+      root.querySelector<HTMLElement>('[data-autofocus]') ?? focusables()[0];
+    autoFocus?.focus();
+
+    const handler = (e: KeyboardEvent) => {
+      if (e.key !== 'Tab') return;
+      const items = focusables();
+      if (items.length === 0) {
+        e.preventDefault();
+        return;
+      }
+      const first = items[0];
+      const last = items[items.length - 1];
+      const current = document.activeElement as HTMLElement | null;
+      if (e.shiftKey && current === first) {
+        e.preventDefault();
+        last.focus();
+      } else if (!e.shiftKey && current === last) {
+        e.preventDefault();
+        first.focus();
+      }
+    };
+
+    root.addEventListener('keydown', handler);
+    return () => {
+      root.removeEventListener('keydown', handler);
+      previouslyFocused?.focus?.();
+    };
+  }, [ref, active]);
+}