feedback7 stats: the local statistics page, ported from PR #323 by @2321Robin as submitted (record store ui/stats.js, viewer screens/stats.js, stats.css, entries on the title / lobby / room screens, 20 tests); golden: unchanged

- public/js/ui/stats.js: one record per finished match in localStorage (sp.pref.stats, envelope {v, records}, at most
  1000, quota shrink of the oldest), content-id dedupe of the lobby's result replay, a MIGRATIONS chain and
  normalizeRecord (defaults for missing fields, unknown fields kept, a newer envelope refused), merge-by-id export /
  import, aggregateStats.
- public/js/screens/stats.js: the viewer (总览 / 策略 / 称号 / 战斗累计 / 最近对局, a row re-views that match's
  settlement), 导出 / 导入 / 清空.
- Applied from the PR's own diff after reading it in full (no network, no new dependency, server untouched); only the
  UI strings of the page are added to the four packs (en from the PR; ja / ko / zh-TW translated for this commit).
- The follow-up commits rework the page (the counting rule for matches left early, storage bounds, the layout).
- Tests: test/ui/stats.test.js 20 / 20; i18n extract 0, check --all --strict clean.

public/index.html

@@ -38,6 +38,7 @@
   <link rel="stylesheet" href="/css/screens/result.css" />
   <link rel="stylesheet" href="/css/screens/guide.css" />
   <link rel="stylesheet" href="/css/screens/loadout.css" />
+  <link rel="stylesheet" href="/css/screens/stats.css" />
   <link rel="stylesheet" href="/css/emotes.css" />
   <link rel="stylesheet" href="/css/devices.css" />