User-facing theme toggle ships now that the design system has
been audited end-to-end. A Sun/Moon button lives between the
threat indicator and the SYSTEM status pill in the topbar — same
slim 28x28 voice as the rest of the topbar controls, no chrome
shouting at the user.
Click coords drive a View Transitions API circle clip-path that
grows from the cursor to the farthest viewport corner over 520ms
with the project's standard --ease curve. Browsers without
startViewTransition (older Firefox, Safari < 18) fall through to
an unanimated swap — the hook returns instantly in that case.
Persistence is two-tier:
- localStorage decnet_theme — the user's saved preference, the
thing the topbar toggle writes. Survives reloads, applies
everywhere.
- sessionStorage decnet_theme_lab — dev-mode lab override (Task
3). Tab-scoped, wins on boot so devs can A/B without nuking
the saved preference.
App.tsx hydrates both on first mount in the right order so the
correct theme is on <html> before the first paint.
useThemeToggle is a small hook in lib/ rather than a Layout-only
helper so the same toggle can be reused later from a settings page
or hotkey.
