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 Add menu/preview-card/group; remove the superseded dropdown-menu,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/ai-elements/attachments.tsx

@@ -2,10 +2,10 @@
 
 import { Button } from "@/components/ui/button";
 import {
-  HoverCard,
-  HoverCardContent,
-  HoverCardTrigger,
-} from "@/components/ui/hover-card";
+  PreviewCard as HoverCard,
+  PreviewCardPopup as HoverCardContent,
+  PreviewCardTrigger as HoverCardTrigger,
+} from "@/components/ui/preview-card";
 import { cn } from "@/lib/utils";
 import type { FileUIPart, SourceDocumentUIPart } from "ai";
 import {

frontend/components/ai-elements/audio-player.tsx

@@ -2,9 +2,9 @@
 
 import { Button } from "@/components/ui/button";
 import {
-  ButtonGroup,
-  ButtonGroupText,
-} from "@/components/ui/button-group";
+  Group as ButtonGroup,
+  GroupText as ButtonGroupText,
+} from "@/components/ui/group";
 import { cn } from "@/lib/utils";
 import type { Experimental_SpeechResult as SpeechResult } from "ai";
 import {

frontend/components/ai-elements/context.tsx

@@ -2,10 +2,10 @@
 
 import { Button } from "@/components/ui/button";
 import {
-  HoverCard,
-  HoverCardContent,
-  HoverCardTrigger,
-} from "@/components/ui/hover-card";
+  PreviewCard as HoverCard,
+  PreviewCardPopup as HoverCardContent,
+  PreviewCardTrigger as HoverCardTrigger,
+} from "@/components/ui/preview-card";
 import { Progress } from "@/components/ui/progress";
 import { cn } from "@/lib/utils";
 import type { LanguageModelUsage } from "ai";

frontend/components/ai-elements/inline-citation.tsx

@@ -8,10 +8,10 @@ import {
   CarouselItem,
 } from "@/components/ui/carousel";
 import {
-  HoverCard,
-  HoverCardContent,
-  HoverCardTrigger,
-} from "@/components/ui/hover-card";
+  PreviewCard as HoverCard,
+  PreviewCardPopup as HoverCardContent,
+  PreviewCardTrigger as HoverCardTrigger,
+} from "@/components/ui/preview-card";
 import { cn } from "@/lib/utils";
 import { ArrowLeftIcon, ArrowRightIcon } from "lucide-react";
 import type { ComponentProps } from "react";

frontend/components/ai-elements/message.tsx

@@ -1,13 +1,10 @@
 "use client";
 
 import { Button } from "@/components/ui/button";
-import {
-  ButtonGroup,
-  ButtonGroupText,
-} from "@/components/ui/button-group";
+import { Group, GroupText } from "@/components/ui/group";
 import {
   Tooltip,
-  TooltipContent,
+  TooltipPopup,
   TooltipProvider,
   TooltipTrigger,
 } from "@/components/ui/tooltip";
@@ -101,10 +98,10 @@ export const MessageAction = ({
     return (
       <TooltipProvider>
         <Tooltip>
-          <TooltipTrigger>{button}</TooltipTrigger>
-          <TooltipContent>
+          <TooltipTrigger render={button} />
+          <TooltipPopup>
             <p>{tooltip}</p>
-          </TooltipContent>
+          </TooltipPopup>
         </Tooltip>
       </TooltipProvider>
     );
@@ -227,7 +224,7 @@ export const MessageBranchContent = ({
   ));
 };
 
-export type MessageBranchSelectorProps = ComponentProps<typeof ButtonGroup>;
+export type MessageBranchSelectorProps = ComponentProps<typeof Group>;
 
 export const MessageBranchSelector = ({
   className,
@@ -241,7 +238,7 @@ export const MessageBranchSelector = ({
   }
 
   return (
-    <ButtonGroup
+    <Group
       className={cn(
         "[&>*:not(:first-child)]:rounded-l-md [&>*:not(:last-child)]:rounded-r-md",
         className
@@ -307,7 +304,7 @@ export const MessageBranchPage = ({
   const { currentBranch, totalBranches } = useMessageBranch();
 
   return (
-    <ButtonGroupText
+    <GroupText
       className={cn(
         "border-none bg-transparent text-muted-foreground shadow-none",
         className
@@ -315,7 +312,7 @@ export const MessageBranchPage = ({
       {...props}
     >
       {currentBranch + 1} of {totalBranches}
-    </ButtonGroupText>
+    </GroupText>
   );
 };
 

frontend/components/ai-elements/open-in-chat.tsx

@@ -2,13 +2,13 @@
 
 import { Button } from "@/components/ui/button";
 import {
-  DropdownMenu,
-  DropdownMenuContent,
-  DropdownMenuItem,
-  DropdownMenuLabel,
-  DropdownMenuSeparator,
-  DropdownMenuTrigger,
-} from "@/components/ui/dropdown-menu";
+  Menu as DropdownMenu,
+  MenuPopup as DropdownMenuContent,
+  MenuItem as DropdownMenuItem,
+  MenuGroupLabel as DropdownMenuLabel,
+  MenuSeparator as DropdownMenuSeparator,
+  MenuTrigger as DropdownMenuTrigger,
+} from "@/components/ui/menu";
 import { cn } from "@/lib/utils";
 import {
   ChevronDownIcon,

frontend/components/ai-elements/prompt-input.tsx

@@ -10,20 +10,20 @@ import {
   CommandSeparator,
 } from "@/components/ui/command";
 import {
-  DropdownMenu,
-  DropdownMenuContent,
-  DropdownMenuItem,
-  DropdownMenuTrigger,
-} from "@/components/ui/dropdown-menu";
+  Menu as DropdownMenu,
+  MenuPopup as DropdownMenuContent,
+  MenuItem as DropdownMenuItem,
+  MenuTrigger as DropdownMenuTrigger,
+} from "@/components/ui/menu";
 import {
-  HoverCard,
-  HoverCardContent,
-  HoverCardTrigger,
-} from "@/components/ui/hover-card";
+  PreviewCard as HoverCard,
+  PreviewCardPopup as HoverCardContent,
+  PreviewCardTrigger as HoverCardTrigger,
+} from "@/components/ui/preview-card";
+import { Button as InputGroupButton } from "@/components/ui/button";
 import {
   InputGroup,
   InputGroupAddon,
-  InputGroupButton,
   InputGroupTextarea,
 } from "@/components/ui/input-group";
 import {

frontend/components/ai-elements/snippet.tsx

@@ -1,9 +1,9 @@
 "use client";
 
+import { Button as InputGroupButton } from "@/components/ui/button";
 import {
   InputGroup,
   InputGroupAddon,
-  InputGroupButton,
   InputGroupInput,
   InputGroupText,
 } from "@/components/ui/input-group";