Migrate UI to COSS components + COSS neutral theme; add i18next

Components:
- Replace components/ui primitives with COSS equivalents from the @coss/* shadcn
  registry. Add menu/preview-card/group; remove the superseded dropdown-menu,
  hover-card, button-group; keep carousel (no COSS equivalent).
- Migrate live call sites to canonical COSS APIs preserving layout/behavior:
  sidebar menus (nav-user, team-switcher, nav-notifications), chat-input radio
  menu, patient dialogs/cards, auth forms (FieldGroup -> flex column), settings.
- ai-elements: migrate the live conversation/message with behavior parity
  (Tooltip via render, Group/GroupText); repoint dormant files (hover-card ->
  preview-card, dropdown-menu -> menu, button-group -> group, InputGroupButton
  -> Button). Residual pre-existing Base UI type drift stays behind
  ignoreBuildErrors.

Theme:
- Adopt COSS default neutral tokens in globals.css with light + dark palettes.
- Add next-themes (defaultTheme=dark, enableSystem) and drop the forced dark
  class. Align font variables to the COSS contract (--font-sans/-heading/-mono).
  Make scrollbars theme-aware.

i18n:
- Add i18next + react-i18next (lib/i18n/config.ts, locales/en/translation.json,
  components/i18n-provider.tsx mounted in layout). Convert auth forms, sidebar
  nav, and settings tabs to useTranslation() as the reference pattern.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -25,13 +25,17 @@
     "cmdk": "^1.1.1",
     "embla-carousel-react": "^8.6.0",
     "framer-motion": "^12.40.0",
+    "i18next": "^26.3.1",
+    "i18next-browser-languagedetector": "^8.2.1",
     "lucide-react": "^1.17.0",
     "media-chrome": "^4.19.0",
     "motion": "^12.40.0",
     "nanoid": "^5.1.11",
     "next": "16.2.6",
+    "next-themes": "^0.4.6",
     "react": "19.2.4",
     "react-dom": "19.2.4",
+    "react-i18next": "^17.0.8",
     "react-jsx-parser": "^2.4.1",
     "shadcn": "^4.8.3",
     "shiki": "^4.1.0",