feedback5 WI: UI strings through t() with an English switch on the title screen and in 设置; pilot screens converted

public/i18n/en.json: Chinese msgid -> English, seeded from PR #70 (@YuriRestia, credited) for every Chinese
literal of the current code (≈830 entries; 966 of the 1 036 literals left for phase 2 already translated),
completed by hand in the official EN terms. public/js/ui/lang.js: boot language (?lang=en|zh, then
localStorage sp.pref.lang, else Chinese; the browser language is not consulted), switchLang, useLang,
LangToggle; main.js renders after initLang and re-renders in place on a switch, and translates server
texts (m.toast / m.ticker frames, room.closed reasons, error codes via ui/toasts.js describeError).
Converted: title screen (English title, wordmark hidden), lobby, room, settings (语言 row, On/Off,
quality, hotkeys), the shell (crash screen, toasts) and DifficultyTag.

tools/i18n.mjs: extract (msgids + untranslated literals per file), check (msgids missing from en.json),
codemod (html`` text / attributes / strings / template literals -> t(), nested strings, N_ for module
level, i18n-ignore lines), seed (fill en.json from a msgid map).

public/js/ui/components.js

@@ -15,6 +15,7 @@ import { h, Fragment } from '../../vendor/preact.module.js';
 import { useEffect, useLayoutEffect, useMemo, useReducer, useRef, useState } from '../../vendor/hooks.module.js';
 import htm from '../../vendor/htm.module.js';
 import { DIFFICULTY_NAMES, DIFFICULTY_COLORS } from '../../../shared/constants.js';
+import { t } from '../../../shared/i18n.js';
 import { serverNow } from '../store.js';
 import { data, useData, localAsset } from '../data.js';
 import { uiUrl } from './assetUrls.js';
@@ -781,7 +782,7 @@ export function DifficultyIcon({ difficulty, class: cls }) {
  * @param {{ difficulty: string, size?: 'sm'|'md'|'lg', class?: string, code?: string }} props
  */
 export function DifficultyTag({ difficulty, size = 'md', class: cls, code }) {
-  const name = DIFFICULTY_NAMES[difficulty] || difficulty || '—';
+  const name = DIFFICULTY_NAMES[difficulty] ? t(DIFFICULTY_NAMES[difficulty]) : difficulty || '—';
   const color = DIFFICULTY_COLORS[difficulty] || 'var(--text-lo)';
   return html`<span class=${cx('dtag', `dtag--${size}`, cls)} style=${`--d-color:${color}`}>
     <${DifficultyIcon} difficulty=${difficulty} class="dtag__icon" />