fix(sw): apply thin popup scrollbar to SW popup bodies

Port the uncommitted HMDesign-SW-Viewer change: SWPopup, SWViewerPanel and
the property window share .hm-sw-property__body, so the body and any
scrolling area inside it now use a thin #9A9284 / #E0DACE scrollbar
(--hm-sw-popup-scrollbar-thumb / -track). Document it in DESIGN_RULES.

packages/sw/src/theme/sw.css

@@ -75,6 +75,10 @@
   --hm-sw-property-label: #000;
   --hm-sw-property-table: #d2c8b4;
   --hm-sw-property-inner-border: #948871;
+  /* Popup scrollbar (every SW popup body: SWPopup, SWViewerPanel, property window). Same as the DWG/DXF viewer
+     panels (thin, HM warm-neutral-mid thumb on warm-neutral-light track). */
+  --hm-sw-popup-scrollbar-thumb: #9a9284;
+  --hm-sw-popup-scrollbar-track: #e0dace;
   /* Property fields (Figma 26:12170 ~ 26:12649): Hover border, editing On, dropdown On, unit colors. */
   --hm-sw-property-field-hover-border-color: rgba(33, 29, 19, 0.4);
   --hm-sw-property-field-editing-background: #0f0d0a;
@@ -1514,6 +1518,8 @@ div.hm-sw-header__menu-row { justify-content: space-between; gap: 8px; padding-l
 .hm-sw-property__close:hover > .hm-sw-property__close-default { visibility: hidden; }
 .hm-sw-property__close:hover > .hm-sw-property__close-hover { visibility: visible; }
 .hm-sw-property__body { display: flex; flex-direction: column; gap: 8px; min-height: 0; overflow-y: auto; overscroll-behavior: contain; padding: 7px 7px 7px; box-shadow: inset 0 2px 2px rgba(255,255,255,.6); }
+/* Popup scrollbar: the body and any scrolling area inside it (zero-specificity so content can still override). */
+.hm-sw-property__body, .hm-sw-property__body :where(*) { scrollbar-width: thin; scrollbar-color: var(--hm-sw-popup-scrollbar-thumb) var(--hm-sw-popup-scrollbar-track); }
 .hm-sw-property[data-selection-kind="all"] .hm-sw-property__body { padding-bottom: 0; }
 .hm-sw-property[data-selection-kind="all"] .hm-sw-property__counts { margin-top: -8px; }
 .hm-sw-property__group { flex: 0 0 auto; }