feat(i18n): add Khmer language support

Register Khmer (km) locale, add complete 1,394-entry dashboard translation,
and show the Cambodia flag in the language selector. Place Khmer immediately
before Thai in the selector and keep the header locale in sync after switching.

src/shared/constants/locales.js

@@ -22,6 +22,7 @@ export const LOCALE_FLAGS = {
   "tl": "🇵🇭",
   "id": "🇮🇩",
   "th": "🇹🇭",
+  "km": "🇰🇭",
   "hi": "🇮🇳",
   "bn": "🇧🇩",
   "ur": "🇵🇰",