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 Add menu/preview-card/group; remove the superseded dropdown-menu,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/switch.tsx

@@ -1,32 +1,30 @@
-"use client"
+"use client";
 
-import { Switch as SwitchPrimitive } from "@base-ui/react/switch"
+import { Switch as SwitchPrimitive } from "@base-ui/react/switch";
+import type React from "react";
+import { cn } from "@/lib/utils";
 
-import { cn } from "@/lib/utils"
-
-function Switch({
+export function Switch({
   className,
-  size = "default",
   ...props
-}: SwitchPrimitive.Root.Props & {
-  size?: "sm" | "default"
-}) {
+}: SwitchPrimitive.Root.Props): React.ReactElement {
   return (
     <SwitchPrimitive.Root
-      data-slot="switch"
-      data-size={size}
       className={cn(
-        "peer group/switch relative inline-flex shrink-0 items-center rounded-full border-2 transition-all outline-none after:absolute after:-inset-x-3 after:-inset-y-2 focus-visible:border-ring focus-visible:ring-3 focus-visible:ring-ring/30 aria-invalid:border-destructive aria-invalid:ring-3 aria-invalid:ring-destructive/20 data-[size=default]:h-5 data-[size=default]:w-11 data-[size=sm]:h-4 data-[size=sm]:w-7 dark:aria-invalid:border-destructive/50 dark:aria-invalid:ring-destructive/40 data-checked:border-primary data-checked:bg-primary data-unchecked:border-transparent data-unchecked:bg-input/90 data-disabled:cursor-not-allowed data-disabled:opacity-50",
-        className
+        "inline-flex h-[calc(var(--thumb-size)+2px)] w-[calc(var(--thumb-size)*2-2px)] shrink-0 items-center rounded-full p-px outline-none transition-[background-color,box-shadow] duration-200 [--thumb-size:--spacing(5)] focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-1 focus-visible:ring-offset-background data-disabled:cursor-not-allowed data-checked:bg-primary data-unchecked:bg-input data-disabled:opacity-64 sm:[--thumb-size:--spacing(4)]",
+        className,
       )}
+      data-slot="switch"
       {...props}
     >
       <SwitchPrimitive.Thumb
+        className={cn(
+          "pointer-events-none block aspect-square h-full origin-left in-[[role=switch]:active,[data-slot=label]:active,[data-slot=field-label]:active]:not-data-disabled:scale-x-110 in-[[role=switch]:active,[data-slot=label]:active,[data-slot=field-label]:active]:rounded-[var(--thumb-size)/calc(var(--thumb-size)*1.1)] rounded-(--thumb-size) bg-background shadow-sm/5 will-change-transform [transition:translate_.15s,border-radius_.15s,scale_.1s_.1s,transform-origin_.15s] data-checked:origin-[var(--thumb-size)_50%] data-checked:translate-x-[calc(var(--thumb-size)-4px)]",
+        )}
         data-slot="switch-thumb"
-        className="pointer-events-none block rounded-full bg-background shadow-sm ring-0 transition-transform not-dark:bg-clip-padding group-data-[size=default]/switch:h-4 group-data-[size=default]/switch:w-6 group-data-[size=sm]/switch:h-3 group-data-[size=sm]/switch:w-4 data-checked:translate-x-[calc(100%-8px)] dark:data-checked:bg-primary-foreground data-unchecked:translate-x-0 dark:data-unchecked:bg-foreground"
       />
     </SwitchPrimitive.Root>
-  )
+  );
 }
 
-export { Switch }
+export { SwitchPrimitive };