feat(web): mount ThemeSwitch in header + pre-paint theme init (#59)

web/src/shell/app-shell.tsx

@@ -4,6 +4,7 @@ import { useTranslation } from "react-i18next";
 import { useLogout } from "../api/queries";
 import { Button } from "@/components/ui/button";
 import { LangSwitch } from "./lang-switch";
+import { ThemeSwitch } from "./theme-switch";
 import { Sidebar } from "./sidebar";
 
 export function AppShell() {
@@ -22,6 +23,7 @@ export function AppShell() {
       <div className="flex flex-1 flex-col">
         <header className="flex items-center gap-4 border-b px-4 py-2">
           <div className="flex-1" />
+          <ThemeSwitch />
           <LangSwitch />
           <Button variant="ghost" size="sm" onClick={onSignOut}>
             {t("auth.signOut")}