Show Chinese locale only when Chinese language signals are detected.

Hide the language switcher for English-only browsers; gate /zh via Accept-Language and a client cookie.

Co-authored-by: Cursor <cursoragent@cursor.com>

messages/en.json

@@ -50,7 +50,8 @@
   "lang": {
     "label": "Language",
     "en": "English",
-    "zh": "中文"
+    "zh": "中文",
+    "zhHint": "Shown when the browser or system prefers Chinese"
   },
   "footer": {
     "offsets": "Flash @ 0x1000 / 0x8000 / 0xe000 / 0x10000",

messages/zh.json

@@ -50,7 +50,8 @@
   "lang": {
     "label": "语言",
     "en": "English",
-    "zh": "中文"
+    "zh": "中文",
+    "zhHint": "仅在检测到浏览器或系统中文时显示"
   },
   "footer": {
     "offsets": "刷写地址 @ 0x1000 / 0x8000 / 0xe000 / 0x10000",