feat(tui): add Mode 2031 theme detection with tri-state appearance mode

[kavhnr]

mode-2031-theme-detection.mp4

Issue for this PR

Closes #13232

Supersedes #13233 (closed to rebase on current dev).

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

OpenTUI v0.1.78 added Mode 2031 dark/light theme detection (opentui#657). OpenCode is already on v0.1.79 (#13036) which includes these APIs, but doesn't use them. This PR integrates renderer.themeMode / renderer.on("theme_mode", ...) into OpenCode's theme system.

Three changes:

1. Tri-state appearance mode (auto/dark/light) — replaces the binary toggle. auto follows OS via Mode 2031, falling back to OSC 11 luminance on unsupported terminals. dark/light are manual overrides.
2. Per-mode theme config — "theme": { "light": "github", "dark": "catppuccin" } alongside existing "theme": "opencode" (backward compatible).
3. Appearance selector in the theme dialog — ←/→ arrow keys cycle modes, clickable, reverts on cancel.

Mode 2031 is the same approach used by Neovim and Helix. Event-driven, zero overhead. Unsupported terminals silently ignore the sequences — no degradation.

Why Mode 2031 over the approaches in existing open PRs:

Approach	PR	Problem
Poll terminal bg every 5s	#7182, #7162	Wastes CPU, delayed detection, races with palette queries
execSync("defaults read") / gsettings every 3s	#9719	Platform-specific, breaks in SSH/containers/tmux, spawns child processes
Mode 2031 (this PR)	—	Terminal-native, event-driven, zero overhead

Edge cases handled:

• Unsupported terminals: detectedMode stays at initial OSC 11 value. No degradation.
• Custom themes without variants: activeThemeMode() falls back to "dark". Existing custom themes work identically.
• Cancel in dialog: both theme and mode revert to initial values.
• Event cleanup: onCleanup(() => renderer.off("theme_mode", ...)) prevents leaks.

How did you verify your code works?

• bun run typecheck passes (16/16 packages, zero errors)
• Tested on Ghostty (Mode 2031 supported): toggling macOS appearance instantly switches theme variants
• Tested on Terminal.app (Mode 2031 unsupported): falls back to existing behavior, manual mode selector works, no errors
• Backward compatible: "theme": "opencode" works, "theme": { "light": "github", "dark": "catppuccin" } works
• All 31 dual-variant built-in themes resolve correctly in both modes
• aura and ayu (single-palette) unaffected by mode changes

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[kavhnr]

@RhysSullivan this is similar to your PR#7162 so might be up your ally to review/hopefully merge for people that use different modes.