Session picker: selected-but-inactive row is indistinguishable from other inactive rows (low contrast)

Describe the bug

In the sessions sidebar (shown via the session picker / /resume view), there are three distinct visual states for a row, but only two are easily distinguishable:

1. Active session (the one currently open) - solid, high-contrast accent-blue background. Clear.
2. Selected/cursor row (the row the keyboard/mouse cursor is on, used to switch-to or close a session, when it isn't the active one) - a subtle dark grey background.
3. Other inactive sessions (not selected, not active) - plain background, no highlight.

The problem: state 2's highlight color is too close in luminance to the plain background (state 3), especially in the default theme: "github" palette. When only two sessions are open (one active, one selected-but-inactive), it's very hard to tell that the second row is actually focused/selected at all.

Steps to reproduce

1. Open Copilot CLI with 2+ saved sessions.
2. Open the sessions view (e.g. /resume, or the sessions sidebar shown in some UI surfaces).
3. Move the selection cursor to a session row that is not the currently active one.
4. Observe that the "selected" row's background is only marginally darker/lighter than an unselected, non-active row.

Expected behavior

The selected-but-inactive row should have a clearly distinguishable background (e.g., a mid-contrast highlight) independent of the active-row accent color and the plain inactive-row background, so all three states are visually unambiguous at a glance - regardless of which of the 5 whole-theme presets (default, github, dim, high-contrast, colorblind) is active.

Additional context

• Checked copilot help config / the full settings reference - there is currently no granular setting to override just this element; only whole-palette theme switching is exposed.
• Suggested labels: area:sessions, area:theming-accessibility
• CLI version: 1.0.79, Windows 11, Windows Terminal, theme: "github" (default)

从 /resume 会话选择器开始，找到行样式以及五个全主题预设：default、github、dim、high-contrast 和 colorblind。在每个预设中，将已选中但未激活的行与其他未激活的行进行比较。完成标准是：无需添加细粒度配置设置，三个会话状态都能在视觉上区分开。