feat(settings): in-app settings panel with live theme and terminal colours

[guillaumejay]

What

A settings panel — ⚙ in the sidebar header, or mod+, (open-settings,
rebindable) — with two pickers:

• Interface theme: dark, light, and four new presets named after the
  terminal schemes (solarized-dark, solarized-light, gruvbox-dark,
  gruvbox-light), mapped to iced's built-in themes.
• Terminal colours: Built-in or one of the four schemes.

A pick applies immediately and saves itself — no Apply button, no
restart:

• the chrome follows on the next frame (Shell::theme reads it every frame);
• every running terminal is recoloured: PtyManager now shares its palette with
  the terminal threads (Arc<RwLock<Palette>>) and set_palette sends each a
  Repaint. OSC 10/11 colour queries answer from the new palette too;
• only the picked key (theme or terminal.colors.scheme) is written; the rest
  of settings.json — explicit colour overrides, unknown keys — is preserved.
  A file that is not a JSON object is left alone rather than rewritten.

The panel and saving go through an injected AppearanceSink port, so tests
neither recolour real terminals nor touch the user's settings.json.

Light themes made usable

• theme now degrades alone on an unknown value (or null, which MCP
  set_option writes to unset) instead of failing serde for the whole file and
  resetting every setting.
• Status dots darken on light surfaces; a test holds every status on every
  light theme to WCAG's 3:1 non-text floor (Busy on Solarized Light was 2.89
  before the tuning).
• The unfocused-window scrim fades toward the grid's own background instead
  of black, which turned a light grid grey.

Known limit

A program reads the terminal's background once at startup to pick a light
or dark look — Claude Code does. A Claude session already running keeps its
look until restarted; the panel says so, and so does the book.

Docs

New book page workspace/settings-panel.md; keyboard reference
(open-settings), settings.md, stdio.md theme values, settings.example.jsonc,
README, sidebar diagram. Roadmap: new F-settings-panel, F-settings updated.

Testing

cargo fmt --check, clippy -D warnings, cargo test --workspace,
check-deps, check-arch, markdownlint, roadmark validate all pass
locally. New tests: settings round-trip and per-key persistence, lenient theme
parse, dot contrast, live repaint of a running PTY session, chord → panel →
escape, pick → repaint/persist via the sink. The overlay escape sweep covers the
new rung.

Not verified on screen: Windows' single-instance mutex prevents running a
second instance beside the one this was developed in. Needs a visual pass.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Br9mLSj4WaKSCWw7noUrAe