fix(theme-toggle): sync theme across tabs on storage event

[yyyr-p]

Summary

ThemeToggle's storage event handler previously just called onStoreChange, which made useSyncExternalStore re-read the DOM. But the DOM was never modified by another tab — a storage event means a peer tab wrote to localStorage, while this tab's DOM state was unchanged, so cross-tab theme switches were silently dropped.

Fix: filter storage events by THEME_STORAGE_KEY, parse the target theme from event.newValue, call applyTheme to sync this tab's dataset.theme / style.colorScheme to that value, then trigger onStoreChange so useSyncExternalStore re-renders. Unknown or missing newValue is ignored; events for other storage keys are untouched; removeEventListener uses the same stable handler reference so no listener leaks.

Admin pages do not render ThemeToggle (SiteHeader is not mounted under src/app/admin/), so this handler never fires in an admin tab and does not conflict with the initial-paint script's forced-light policy on `/admin`.

Test plan

• `npm run typecheck` passes
• `npm run lint` passes (1 pre-existing unrelated warning)
• `npm test` passes
• `npm run build` passes
• Manual: open two non-admin tabs, toggle theme in one, other tab mirrors immediately (both dark→light and light→dark)