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 Add menu/preview-card/group; remove the superseded dropdown-menu,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/components/ui/separator.tsx

@@ -1,25 +1,23 @@
-"use client"
+import { Separator as SeparatorPrimitive } from "@base-ui/react/separator";
+import type React from "react";
+import { cn } from "@/lib/utils";
 
-import { Separator as SeparatorPrimitive } from "@base-ui/react/separator"
-
-import { cn } from "@/lib/utils"
-
-function Separator({
+export function Separator({
   className,
   orientation = "horizontal",
   ...props
-}: SeparatorPrimitive.Props) {
+}: SeparatorPrimitive.Props): React.ReactElement {
   return (
     <SeparatorPrimitive
+      className={cn(
+        "shrink-0 bg-border data-[orientation=horizontal]:h-px data-[orientation=horizontal]:w-full data-[orientation=vertical]:w-px data-[orientation=vertical]:not-[[class^='h-']]:not-[[class*='_h-']]:self-stretch",
+        className,
+      )}
       data-slot="separator"
       orientation={orientation}
-      className={cn(
-        "shrink-0 bg-border data-horizontal:h-px data-horizontal:w-full data-vertical:w-px data-vertical:self-stretch",
-        className
-      )}
       {...props}
     />
-  )
+  );
 }
 
-export { Separator }
+export { SeparatorPrimitive };