undo: header buttons, shortcuts, wiring

- App provides HistoryHandle as context (both build modes; stacks stay
  empty on the server) and passes it to persist::init_hydrated
- persist: init_hydrated starts undo tracking after the localStorage
  load (first captured state = saved state, not mock); clear_all and
  apply_import call undo::before_replace so they are undoable
- header: Отменить/Вернуть buttons with disabled states driven by the
  reactive stack depths; Ctrl+Z / Ctrl+Shift+Z / Ctrl+Y via a window
  keydown listener that skips editable targets (browser owns those)
- SCSS: disabled control buttons dim out

style/_header.scss

@@ -59,6 +59,11 @@ header {
             line-height: 1em;
             margin: 2px 5px;
             @include mixins.button;
+
+            &:disabled {
+                opacity: 0.45;
+                cursor: default;
+            }
         }
     }