fix(tui): improve light-mode selection contrast

[fcoury-oai]

Why

On light terminal backgrounds, selected rows in several TUI pickers were rendered with the same bright cyan accent used on dark themes. Against the light menu surface, that made the current selection hard to distinguish at a glance.

Before

After

What changed

• Added a shared background-aware accent style for active/selected TUI controls.
• Use a darker cyan-family accent on light backgrounds while preserving the existing bright cyan accent on dark or unknown backgrounds.
• Reused that accent across shared picker rows and the custom selection-like surfaces that had drifted separately: picker tabs, hooks browsing, external-agent migration choices, and /keymap affordances.
• Added focused tests for the light/dark accent rule and rendered selected-row styling.

How to Test

1. Start Codex in a terminal using a light background theme.
2. Type / to open the slash-command picker and move the selection through a few rows.
3. Confirm that the selected row is visibly colored with strong contrast instead of blending into the popup surface.
4. Open /keymap and confirm the active tab, selected rows, and picker hint accents use the same light-theme accent treatment.
5. In a dark terminal theme, repeat the slash-picker check and confirm the existing bright cyan selection styling is preserved.

Targeted tests:

• cargo test -p codex-tui accent_style_uses_
• cargo test -p codex-tui selected_rows_use_the_shared_accent_style
• cargo test -p codex-tui selected_event_rows_use_the_shared_accent_style

Notes:

• A full cargo test -p codex-tui run reached the end of the suite but hit an unrelated existing stack overflow in tests::fork_last_filters_latest_session_by_cwd_unless_show_all.

[etraut-openai]

Looks good! I was able to repro the bug on iTerm2 with a light theme, and I confirmed that the fix addresses the problem.