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)

Bắt đầu từ trình chọn phiên của /resume và xác định kiểu hiển thị của các hàng cùng năm preset cho toàn bộ theme: default, github, dim, high-contrast và colorblind. So sánh các hàng được chọn nhưng không hoạt động với các hàng không hoạt động khác trong từng preset. Hoàn thành khi cả ba trạng thái phiên đều có thể được phân biệt bằng mắt mà không thêm một thiết lập cấu hình chi tiết.