chore(i18n): remove dead rewards.* keys from zh-CN

`rewards.badge` and `rewards.refresh` are defined only in zh-CN.ts,
have no counterparts in en.ts, and are not referenced anywhere in
source. Existing guard tests at apps/web/tests/home.test.tsx:322-323
and tests/web/home.test.tsx:402-403 already assert these raw keys
never appear in rendered markup — confirming they are dead
translations.

After this change, en.ts and zh-CN.ts both expose exactly 1052 keys,
restoring full locale parity.

Author: pftom

apps/web/src/i18n/locales/zh-CN.ts

@@ -286,10 +286,8 @@ const zhCN = {
   "home.noRecentActivity": "暂无最近对话",
 
   // ── Rewards ──
-  "rewards.badge": "奖励中心",
   "rewards.title": "分享 nexu，获取额外积分",
   "rewards.desc": "把 nexu 分享给你的社区，完成任务获取额外积分。",
-  "rewards.refresh": "刷新",
   "rewards.progressLabel": "已完成",
   "rewards.earnedLabel": "已获得",
   "rewards.totalCredits": "奖励额度 {{n}}",