fix(sw): rounded menu surface end before the viewer strip; solid viewer background

User requests (2026-10-21), CSS and docs only; no Props, public .d.ts unchanged.
- The viewer menu surface ends before the trailing strip like the SWHeader object area:
  6px right radius (--hm-sw-header-object-radius), the same face and border, a 2px
  header gap. Painted by the first strip segment over the empty group padding + gap
  (4px more room below 1180px); heights, edges and button positions unchanged.
- --hm-sw-viewer-background: the Storybook viewer canvas solid #0a0a0b instead of the
  gradient (viewer header only; SWHeader, Footer, Popup unchanged).

packages/sw/ai/DESIGN_RULES.md

@@ -141,7 +141,7 @@
 
 | 영역 | 토큰 = 기본값 |
 |---|---|
-| Header | `--hm-sw-viewer-height` 52px, `--hm-sw-viewer-background` `linear-gradient(180deg, #266651 0%, #123328 45%, #0a221d 100%)`, `--hm-sw-viewer-border-color` #071e18, `--hm-sw-viewer-text` #fff, `--hm-sw-viewer-muted` #bddfd4, `--hm-sw-viewer-accent` #ffc600, `--hm-sw-viewer-hover` #266651 |
+| Header | `--hm-sw-viewer-height` 52px, `--hm-sw-viewer-background` #0a0a0b(단색, 2026-10-21 — Storybook 뷰어 캔버스 바탕색), `--hm-sw-viewer-border-color` #071e18, `--hm-sw-viewer-text` #fff, `--hm-sw-viewer-muted` #bddfd4, `--hm-sw-viewer-accent` #ffc600, `--hm-sw-viewer-hover` #266651 |
 | 로고 면 | `--hm-sw-viewer-brand-width` 72px, `--hm-sw-viewer-brand-empty-width` 24px(로고 없음), `--hm-sw-viewer-brand-top` #123328, `--hm-sw-viewer-brand-middle` #205241, `--hm-sw-viewer-brand-bottom` #071e18 |
 | 크기·글자 | `--hm-sw-viewer-gap` 4px, `--hm-sw-viewer-radius` 4px, `--hm-sw-viewer-control-height` 40px, `--hm-sw-viewer-button-height` 34px, `--hm-sw-viewer-font-size` 14px, `--hm-sw-viewer-label-size` 11px, `--hm-sw-viewer-icon-size` 16px, `--hm-sw-viewer-disabled-opacity` 0.6 |
 | Header 스위치·아이콘 | `--hm-sw-viewer-switch-off` #0a221d, `--hm-sw-viewer-switch-bevel` `inset 0 1px 0 rgba(255, 255, 255, 0.1), inset 0 -1px 1px rgba(0, 0, 0, 0.25)`, `--hm-sw-viewer-icon-outline-color` = `var(--hm-sw-header-text-outline-color)`, `--hm-sw-viewer-icon-outline-width` 1px |