feat: add the light palette, and the module that resolves a theme

The dark tokens re-struck under `[data-theme="light"]`. Nothing sets that
attribute yet, so the app still renders exactly as it did.

Two things could not be tokens. Game hues are data — ours and the
reader's — and every one was picked against a near-black ground, so
`readableHue` darkens them along the same hue until they read on paper,
and returns dark untouched by construction. And the attribute has to be
on the root before React can mount, or a reader who chose light gets a
dark flash on every load, so the shell reads the preference itself.

That leaves the ground colour written in three files that cannot import
each other, which is what the test pins together.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/client/components/Welcome.tsx

@@ -119,7 +119,7 @@ export function Welcome({
           type="button"
           disabled={chosen.length === 0}
           onClick={() => onConfirm(chosen, view)}
-          className="rounded-xl bg-ink px-5 py-3 text-sm font-semibold text-ground transition-colors duration-150 hover:bg-white disabled:cursor-not-allowed disabled:bg-raised disabled:text-faint"
+          className="rounded-xl bg-ink px-5 py-3 text-sm font-semibold text-ground transition-colors duration-150 hover:bg-ink-strong disabled:cursor-not-allowed disabled:bg-raised disabled:text-faint"
         >
           {chosen.length === 0
             ? "Pick at least one game"