fix(theme): 用整页快照过渡消除明暗割裂

- 支持 View Transition 时整页旧/新快照交叉淡入 160ms，背景图与底色不再各自过渡
- 不支持或减少动画偏好时回退到原来的颜色过渡

stash-web/src/AppNext.tsx

@@ -308,8 +308,26 @@ export default function AppNext() {
   useEffect(() => {
     const root = document.documentElement
     if (root.dataset.scheme === scheme) return
+
+    const apply = () => {
+      root.dataset.scheme = scheme
+    }
+    const doc = document as Document & {
+      startViewTransition?: (callback: () => void) => unknown
+    }
+    const reduceMotion = window.matchMedia('(prefers-reduced-motion: reduce)').matches
+
+    if (!reduceMotion && typeof doc.startViewTransition === 'function') {
+      try {
+        doc.startViewTransition(apply)
+        return
+      } catch {
+        // 浏览器正在跑另一段过渡时退回 CSS 过渡
+      }
+    }
+
     root.classList.add('theme-transition')
-    root.dataset.scheme = scheme
+    apply()
     const timer = window.setTimeout(() => root.classList.remove('theme-transition'), 180)
     return () => {
       window.clearTimeout(timer)