chore: update custom themes for new webapp surfaces

Bring the ASCII, Windows 95 and light themes up to date with the
webapp changes since the last theme refresh:

- Flatten/bevel the new Telegram notifications banner icon badge.
- Square off the premium-server and referral-tariff dropdown help
  pills and bevel the standalone referral surfaces (win95).
- Fix the selected-language check vanishing on the highlighted row
  (black on ASCII, inverted bitmap on win95).
- Square the login-screen language trigger and render flag emoji
  monochrome in the ASCII console palette.
- Add a soft drop shadow to the banner on the light theme.

Bump assets_version for all three CSS themes and update the theme
config tests accordingly.

backend/bot/app/web/themes/light/theme.json

@@ -9,7 +9,7 @@
   "use_primary_accent": true,
   "use_in_admin": true,
   "css_file": "style.css",
-  "assets_version": 3,
+  "assets_version": 4,
   "tokens": {
     "color_scheme": "light"
   }