Step 1 of the planned vue->react migration. The standalone
subscription page (sub/sub.go renders the HTML host; React mounts
into #app) is the first entry off vue.
Introduces two shared pieces both entries (and future ones) will
use:
* src/hooks/useTheme.tsx — React Context + useTheme hook + the
same buildAntdThemeConfig (dark/ultra-dark token overrides) and
pauseAnimationsUntilLeave helper the vue version exposes. Same
localStorage keys (dark-mode, isUltraDarkThemeEnabled) and DOM
side effects (body.className, html[data-theme]) so the two stay
in sync across the coexistence period.
* src/i18n/react.ts — i18next + react-i18next loader that reads
the same web/translation/*.json files via import.meta.glob. The
vue-i18n setup in src/i18n/index.js is untouched and still serves
the remaining vue entries.
SubPage.tsx mirrors the vue version's behavior: reads
window.__SUB_PAGE_DATA__ injected by the Go sub server, renders QR
codes / descriptions / Android+iOS deep-link dropdowns, supports
theme cycle and language switch. Uses AntD v5 idioms: Descriptions
items prop, Dropdown menu prop, Layout.Content.
