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);
+}