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/package.json

@@ -21,7 +21,6 @@
     "@xterm/xterm": "^6.0.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
-    "flag-icons": "^7.5.0",
     "gsap": "^3.15.0",
     "leva": "^0.10.1",
     "lucide-react": "^0.577.0",