Adds the settings surface + storage plumbing swarm-ui has been missing:
nowhere to put a client-local preference and no shared code for one to
build on. Scoped small per explicit direction ("small thing somewhere",
localStorage, theme and motion in scope for now) rather than a full
/settings route + nav entry for two toggles.
- frontend/packages/swarm-ui/src/lib/settings-storage.ts: generic
useLocalSetting<T>(key, fallback) hook - read once, write through,
stay in sync with other same-tab consumers of the same key via a
small module-level pub/sub (localStorage's own storage event only
fires cross-tab).
- frontend/packages/swarm-ui/src/lib/theme-apply.ts: tri-state
system/light/dark override, applied by setting the 16 base16 custom
properties inline on <html> (an inline style always outranks a
stylesheet rule, including a media-query-gated one) - colors.css's
own comment on its light-mode block already named this as the
intended mechanism for a future override.
- frontend/packages/swarm-ui/src/lib/motion-apply.ts: tri-state
system/reduce/allow override, applied as a data-motion attribute.
Currently inert - swarm-ui has zero CSS animations yet - included
because the marginal cost riding alongside the theme override is
near zero and it was named in the same scoping answer; the first
swarm-ui animation's own CSS is what makes this do anything.
- frontend/packages/swarm-ui/src/shell/SettingsMenu.{tsx,css}: a
header icon-button + popover holding both selects, same shape as
LinksMenu (manages its own state, not a ui/ primitive, hence no
ComponentsPage demo - same exception LinksMenu already established).
- Shell.tsx/.css: mounts the two override-application hooks once
(every route renders through one Shell), and wraps SettingsMenu +
LinksMenu in a single .shell-header-actions flex wrapper so one
margin-left: auto pushes both to the right edge together - two
adjacent auto-margins on separate elements split the space between
them instead of sitting flush.
Verified the override actually outranks the media query, not just
"looks right": seeded localStorage with each override value while
forcing the opposite OS-level prefers-color-scheme via headless
chromium, both directions render the stored override, not the forced
OS preference. Typecheck and build clean.
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