feat(app): add system appearance following — auto dark/light theme switching

[griffinwork40]

Auto dark/light theme switching

Adds system appearance following so Umber automatically switches between a dark and light palette when macOS toggles Light/Dark mode. The feature is opt-in via "preset": "auto" in the theme config — all existing pinned-preset behaviour is completely unchanged.

Config shape

{
  "theme": {
    "preset": "auto",
    "dark": "classic-repaired",
    "light": "afk-light"
  }
}

Both dark and light can be any shipped preset name. Omitting either falls back to classic-repaired (dark) or afk-light (light). Existing config like "preset": "umber" continues to work identically — no observer fires, the appearance pin in SpaceWindowController is unchanged.

What switches

When the system appearance changes and auto mode is active:

• Terminal theme — the full palette (background, foreground, ANSI 16 colours, cursor, selection)
• Window appearance — derived from the new theme's background luminance via AppConfig.appearance, so the sidebar, titlebar, scroller knob, and selection pill all follow automatically
• Tab strip chrome — background and foreground recalculated from the new theme

Implementation

New file: AppearanceObserver.swift (124 LOC)

KVO on NSApp.effectiveAppearance. The observer is a @MainActor final class held by AppDelegate for the app's lifetime. When the appearance changes, applyThemeSwitch() iterates SpaceWindowController.open, checks each Space's config.autoTheme, and calls space.apply(config:) + window chrome updates — the same fan-out reloadConfig has always done.

Uses bestMatch(from:) to handle all four dark-family appearance variants (darkAqua, vibrantDark, accessibilityHighContrastDarkAqua, accessibilityHighContrastVibrantDark) rather than a string comparison.

Config.swift

• ThemeSpec gains dark: String? and light: String? Decodable sub-fields
• AppConfig gains autoTheme: (dark: Theme, light: Theme)? — nil for all pinned presets
• AppConfig.load() branches on preset == "auto", calls resolveAutoTheme, picks initial theme from AppearanceObserver.currentIsDark
• load() is now @MainActor because it reads NSApp.effectiveAppearance in auto mode; all callers (AppDelegate) were already on the main actor

Config+Theme.swift

resolveAutoTheme(dark:light:warnings:) — same fail-soft contract as resolveTheme: a bad preset name appends a warning and falls back to the default rather than returning a partial result or throwing.

AppDelegate.swift

let appearanceObserver = AppearanceObserver() — one line, held for app lifetime.

StarterConfig.swift

Documents the auto mode with an inline example and the default values for omitted sub-fields.

Constraints

Constraint	Status
No file over 350 LOC	✅ Config.swift at 344, all others clear
Pinned-preset path unchanged	✅ SpaceWindowController not touched
@MainActor throughout	✅ AppearanceObserver, currentIsDark, load()
No try!, no runtime force-unwraps	✅
Config parsing fails soft	✅ bad dark/light name warns + falls back
swift build clean	✅ 0 errors, 0 new warnings
check-file-size.sh passes	✅

What is NOT done

• Color overrides in auto mode (background, foreground, cursor, ansi) — these are silently ignored when preset is "auto". Could be added per-sub-field later if wanted; for now the StarterConfig doc notes they are not supported.
• Per-Space auto vs. pinned mix — each Space carries its own resolved AppConfig, so a Space opened before a ⌘R reload with auto mode, and another opened with a pinned preset, coexist correctly. The observer only acts on Spaces with non-nil autoTheme.
• Smooth cross-fade — the switch is instant (same frame as the system switch), which is consistent with how reloadConfig works.