fix(dashboard): remove country flags from language picker (#29997)

Closes #29750. Reporter flagged that 繁體中文 displayed the TW flag
instead of the PRC flag. Rather than picking a side, drop the
language-flag pairings entirely — languages aren't countries
(English ≠ GB, Portuguese ≠ PT, Mandarin variants ≠ any single
jurisdiction), and endonyms are unambiguous.

- LOCALE_META: strip flagCountryCode field
- LanguageSwitcher: remove LocaleFlagIcon component + both call sites
- main.tsx: drop flag-icons CSS import
- package.json: uninstall flag-icons

web/src/main.tsx

@@ -1,6 +1,5 @@
 import { createRoot } from "react-dom/client";
 import { BrowserRouter } from "react-router-dom";
-import "flag-icons/css/flag-icons.min.css";
 import "./index.css";
 import App from "./App";
 import { SystemActionsProvider } from "./contexts/SystemActions";