fix: show session delete button after the list shifts under the cursor

[killersteps]

Problem

Rename and delete on a session row only appear while that row is hovered. Hover is tracked with JS mouseenter.

After you delete a session, the row below it moves up into the same place. The pointer often has not moved. Browsers do not fire mouseenter (and often do not update :hover) when layout moves an element under a stationary pointer, so the row now under the cursor has hovered === false.

The delete button is then missing until the pointer leaves the row and comes back. This is intermittent: it happens when you delete one session and immediately hover the next one, or when the next row is already under the cursor.

Solution

Keep the existing per-row mouseenter / mouseleave behavior, and additionally:

1. Record the pointer position on the session list.
2. After the list reflows, hit-test row geometry (data-session-id) and treat the row under the cursor as hovered.
3. Ignore pointerleave / mouseleave when the coordinates are still inside the list. Unmounting the deleted row synthesizes leave even though the cursor never left.

Tests

• components/SessionSidebar.test.mjs asserts the geometry probe, the synthetic-leave guard, and showHover = hovered || pointerActive.

node --experimental-strip-types --test components/SessionSidebar.test.mjs passes.

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问题

会话行的重命名 / 删除按钮只在悬停时出现，悬停状态靠 JS 的 mouseenter。

删掉一条会话后，下面那一行会顶上来，占原来的位置。这时指针往往没动。元素是被布局挪到指针下面的，浏览器不会触发 mouseenter（:hover 也经常不更新），于是现在指针底下的那一行 hovered === false。

删除按钮就不会出现，直到指针先离开再移回来。这是偶现的：删掉一个以后立刻悬停到下一条，或者下一条已经在指针下面时，就会踩中。

解决方案

保留原有的逐行 mouseenter / mouseleave，并额外：

1. 在会话列表上记录指针位置。
2. 列表回流后，按行几何（data-session-id）做命中检测，把指针下面的行当作已悬停。
3. 坐标仍在列表内时忽略 pointerleave / mouseleave。被删的那一行卸载时会合成 leave，但指针其实没离开列表。

测试

• components/SessionSidebar.test.mjs 覆盖几何探测、合成 leave 防护，以及 showHover = hovered || pointerActive。

node --experimental-strip-types --test components/SessionSidebar.test.mjs 通过。