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 Add menu/preview-card/group; remove the superseded dropdown-menu,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

@@ -25,12 +25,12 @@ import {
 
 import { PatientFormDialog } from "@/components/chat/patient-form-dialog";
 import {
-  DropdownMenu,
-  DropdownMenuContent,
-  DropdownMenuRadioGroup,
-  DropdownMenuRadioItem,
-  DropdownMenuTrigger,
-} from "@/components/ui/dropdown-menu";
+  Menu,
+  MenuPopup,
+  MenuRadioGroup,
+  MenuRadioItem,
+  MenuTrigger,
+} from "@/components/ui/menu";
 import { cn } from "@/lib/utils";
 
 type ChatInputProps = {
@@ -101,8 +101,8 @@ function SelectPill({
   const selected = options.find((option) => option.value === value);
 
   return (
-    <DropdownMenu>
-      <DropdownMenuTrigger
+    <Menu>
+      <MenuTrigger
         render={
           <button aria-label={ariaLabel} className={triggerClassName} type="button" />
         }
@@ -113,17 +113,17 @@ function SelectPill({
         ) : null}
         <span className="truncate">{selected?.label}</span>
         <ChevronDown className={chevronClassName} />
-      </DropdownMenuTrigger>
-      <DropdownMenuContent align={align}>
-        <DropdownMenuRadioGroup onValueChange={onValueChange} value={value}>
+      </MenuTrigger>
+      <MenuPopup align={align}>
+        <MenuRadioGroup onValueChange={onValueChange} value={value}>
           {options.map((option) => (
-            <DropdownMenuRadioItem key={option.value} value={option.value}>
+            <MenuRadioItem key={option.value} value={option.value}>
               {option.label}
-            </DropdownMenuRadioItem>
+            </MenuRadioItem>
           ))}
-        </DropdownMenuRadioGroup>
-      </DropdownMenuContent>
-    </DropdownMenu>
+        </MenuRadioGroup>
+      </MenuPopup>
+    </Menu>
   );
 }