feat(managers): clickable links in descriptions + real SVG country flags

- linkify: bare http(s) URLs inside channel descriptions become clickable links
  (new tab, rel=noopener noreferrer) — new lib/linkify.tsx, used in the shared
  ChannelAboutContent so both the About overlay and the channel detail page get it
- Country flag: the flag EMOJI never renders on Windows/Chrome (showed the small-caps
  'hu' fallback), so swap it for a real self-hosted SVG flag via the flag-icons package;
  applies to the About overlay + the channel detail page

frontend/package.json

@@ -14,6 +14,7 @@
     "@tanstack/react-query": "^5.51.0",
     "@tanstack/react-virtual": "^3.14.3",
     "clsx": "^2.1.1",
+    "flag-icons": "^7.5.0",
     "hls.js": "^1.6.16",
     "i18next": "^23.11.5",
     "lucide-react": "^0.408.0",