fix(tasks): 扫光改为整条完整扫过 + 折叠/展开按钮分向

用户反馈两条：

1) 扫光只有「部分」：第一版是 45% 宽的窄带 + background-position 位移，
   亮带始终只覆盖小条的一部分（峰值 22.5%→77.5%，两端扫不到），看起来像
   中间游走的一块光斑。改为与小条等宽的渐变带 + transform
   translateX(-100%) → translateX(100%)：光带从左外侧进入、中途覆盖整条、
   再向右外侧离开，行程用条宽本身表达（任意条宽成立），末 1/3 周期停在
   右侧外侧留白。theme.spec 新增守护钉住该形状（宽度 100%、transform 行程、
   不再有 background-position/size 位移）。

2) 折叠按钮与展开按钮长得一样：三处控件原本共用向下箭头（控制条的折叠开关
   更是无论折叠还是展开都画同一个清单图标）。现按动作方向区分——
   卡片小条的逐节点按钮 = chevron up（收进聚合），聚合卡片 = chevron down
   （展开），控制条开关随状态在两者之间切换（并复用同一个折叠图标）。
   组件守护见 tasks-page.spec 的「draws COLLAPSE and EXPAND with different
   glyphs」。

真机证据（3080，真实运行中的后台任务）：242px 小条上每 90ms 采样 ::after 的
matrix 平移量得到 [-23,32,85,131,163,-139,-94,-46,9,64,114,152,179,188]
（左侧 -139 → 右侧 +188，跨度超过一个条宽，单调推进）；三处 svg 比对
expandVsCollapse / clusterMatchesExpand / clusterFlips / clusterCollapseMatchesCard
全部为 true。

验证：typecheck / lint ✅；pnpm test 1256 passed / 9 skipped（新增 2 例）。

(cherry picked from commit 0bb8228e6a7acb4302d9a8a9881d2df04a285152)

src/client/TasksGraph.tsx

@@ -30,7 +30,8 @@ import {
 } from 'react'
 import clsx from 'clsx'
 import {
-  Button, IconChecklistOutlineRegular, IconChevronDownOutlineRegular, IconFullscreenOutlineRegular,
+  Button, IconChevronDownOutlineRegular, IconChevronUpOutlineRegular,
+  IconFullscreenOutlineRegular,
   IconLoadingOutlineRegular, IconTreeCornerRegular,
 } from '@deepseek-ai/dsh-client-ui-primitives'
 import type { TasksAgentNode, TasksFoldNode, TasksNode, TasksWorkflowNode } from './tasks-model.ts'
@@ -164,6 +165,14 @@ export function ViewModeToggle(props: {
 }
 
 /** The fold toggle (expand / re-collapse the settled aggregates). */
+/**
+ * The cluster's fold switch. Its glyph names the action it will take, not the
+ * state it is in: while the settled nodes are folded the button offers to
+ * EXPAND (chevron down), and while they are open it offers to FOLD (chevron
+ * up, the same collapse direction the cards' own bar buttons wear). The old
+ * single checklist glyph read identically in both states, which is what the
+ * reader reported as "折叠按钮和展开一样".
+ */
 export function FoldToggleButton(props: { folded: boolean; onToggleFold(): void }): ReactNode {
   const { folded, onToggleFold } = props
   return (
@@ -171,7 +180,9 @@ export function FoldToggleButton(props: { folded: boolean; onToggleFold(): void
       variant="ghost"
       size="sm"
       className={clsx(css.controlBtn, folded && css.controlBtnActive)}
-      icon={<IconChecklistOutlineRegular size={13} />}
+      icon={folded
+        ? <IconChevronDownOutlineRegular size={13} />
+        : <IconChevronUpOutlineRegular size={13} />}
       aria-pressed={folded}
       aria-label={t(folded ? 'tasksFoldExpand' : 'tasksFoldCollapse')}
       title={t(folded ? 'tasksFoldExpand' : 'tasksFoldCollapse')}