fix: raven onboard text invisible on light-background terminals

[0xKT]

Summary

raven onboard is unreadable on a light-background (white) terminal. Colored text
elements -- unselected list options, the highlighted/current row, free-text input, the
accent-yellow titles/pointer, and one panel subtitle -- render at near-zero contrast and
effectively vanish. Only text with no explicit color (bold, inheriting the terminal's
default foreground) stays visible.

Root cause: the CLI onboarding palette is hardcoded for a dark background and there is
no light/dark theme detection anywhere in raven/. Most people run dark terminals, so
the hardcoded light foregrounds work for them; a colleague on a light iTerm2 profile is the
only one who hits it. This is the CLI (Python: rich + questionary) counterpart of the
TUI-side light-theme work in #79 -- a separate codebase that never got the same treatment.

Steps to reproduce

1. macOS iTerm2 with a light/white background profile (or any light-background terminal).
2. Run raven onboard.
3. Observe the language step and the "Step 1/5 - choose LLM provider" step.

Expected behavior

All onboard text -- list options, the current/highlighted row, input text, titles, and
subtitles -- is legible on a white/near-white terminal background, the same way it is on a
dark background.

Actual behavior

The following elements are near-invisible on white. WCAG contrast ratios (AA body text
needs >= 4.5:1) make the mechanism exact -- every hardcoded color is tuned for a dark
background and fails on white, while the only survivor is the color-less bold text:

Element	Color	Location	On white	On dark #1e1e1e
Unselected list options	fg:#FFF5EA (text)	raven/cli/_styles.py:40	1.08 (fail)	15.48 (ok)
Highlighted/current row	fg:#FFF5EA (highlighted)	raven/cli/_styles.py:27	1.08 (fail)	15.48 (ok)
Free-text input	fg:#FFF5EA (text)	raven/cli/_styles.py:40	1.08 (fail)	15.48 (ok)
Accent titles / pointer / progress dots	#fbe23f	_styles.py:17,22 + markup in onboard_commands.py	1.31 (fail)	12.74 (ok)
Multi-select chosen value	fg:#c8a900 (selected)	raven/cli/_styles.py:30	2.30 (fail)	7.25 (ok)
Bold, no color (visible)	terminal default fg	_styles.py:18	21.00 (ok)	1.26 (fail)

A clean natural experiment sits in a single panel: onboard_commands.py:188-189 renders the
English subtitle as [bold white] and the Chinese subtitle as [dim]. On white, white
foreground is invisible (~1.0:1) while [dim] dims the default black foreground to a
still-readable gray -- which is exactly the reported symptom (English line gone, Chinese
line visible).

Note: highlighted (_styles.py:27) uses noreverse, which deliberately turns off
prompt_toolkit's reverse-video highlight in favor of a light foreground. On a light
background that removes the only background-based selection cue, so the current row has no
visible marker at all.

Environment

• OS: macOS
• Terminal: iTerm2 with a light/white background profile
• Python: 3.12
• Raven: main @ 6079951
• Note: not version-specific -- reproduces on any light-background terminal,
  since the palette is hardcoded regardless of terminal type or version.

Logs or screenshots

Reporter screenshots show the language step and the "Step 1/5" provider step with titles,
options, and subtitles washed out to near-white/near-yellow on a white background. (Attach
on filing.)

Suggested scope (for discussion, not a decision)

• Short-term: stop hardcoding light foregrounds -- restore a background-based highlight for
  the current row (drop noreverse or use a selection background), and avoid white /
  #FFF5EA for text that must read on any background.
• Proper fix: give the CLI a light/dark aware palette, mirroring the TUI approach in
  ui-tui/src/theme.ts (OSC 11 background probe + luminance threshold + env overrides,
  fallback dark), since TS/Python cannot share code but the color table and decision logic
  can be ported. Sibling of Rework light theme readability and contrast #79 (TUI side).

Keep dark-terminal behavior unchanged.

---

Confirming this on the released v0.1.6 (the report tested main @ 6079951), so the broken palette ships in a tagged release, not just main. Same class of terminal as the reporter — iTerm2 with a light/white-background profile, macOS 15.6 (arm64). Adding the actual screenshots, which line up with the contrast table exactly.

1. Language step (raven onboard, before a language is picked)

• Title Raven setup (accent yellow) and the ❯ English pointer/option render washed-out; 中文 [简体] is near-invisible.
• The natural experiment from onboard_commands.py:188-189 is visible directly: the English subtitle ("Let's set up Raven — First, choose your language.") has vanished while the Chinese subtitle ("开始配置 Raven — 请先选择语言。") stays readable — [bold white] gone, [dim] survives, exactly as described.

2. Step 1/5 — choose LLM provider (Chinese mode)

• The accent-yellow title, the 步骤 1/5 header, and the panel subtitle (选择 LLM 服务商) are washed out / effectively gone.
• The entire provider list is invisible — OpenRouter / OpenAI / Anthropic / Gemini / DeepSeek / GitHub Copilot / Codex / 其他 / 返回, including the highlighted current row (❯ OpenRouter…). With noreverse there is no background cue, so there is no visible selection marker at all.
• The only things that remain legible are the color-less bold labels — again matching "bold, no color (visible)" in the table.

So this reproduces on a second machine and on a shipped release; the two screens above are the language step and the Step 1/5 provider step the report references.