fix: settings mobile layout, silent failure on language save

- Settings sidebar was a fixed-width vertical list with no responsive
  breakpoint, squeezing the form into a sliver on mobile. Turn it into
  a horizontal scrollable tab bar under md, vertical sidebar above.
- setLocale() fired the PATCH and swallowed the result with
  .catch(() => {}) — never checked res.ok, no success/failure feedback.
  A failed save (expired session, network blip) looked identical to a
  successful one. Now returns a boolean, reverts local state on
  failure, and the settings form toasts success/failure.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/lib/i18n/provider.tsx

@@ -14,8 +14,8 @@ const STORAGE_KEY = "epicure-locale";
 
 const LocaleContext = createContext<{
   locale: Locale;
-  setLocale: (locale: Locale) => void;
-}>({ locale: "en", setLocale: () => {} });
+  setLocale: (locale: Locale) => Promise<boolean>;
+}>({ locale: "en", setLocale: async () => false });
 
 export function useLocale() {
   return useContext(LocaleContext);
@@ -45,15 +45,27 @@ export function I18nProvider({
     }
   }, [messages, initialLocale]);
 
-  function setLocale(next: Locale) {
+  async function setLocale(next: Locale): Promise<boolean> {
+    const previous = locale;
+    const previousMessages = currentMessages;
     setLocaleState(next);
     setCurrentMessages(messages[next]!);
     localStorage.setItem(STORAGE_KEY, next);
-    fetch("/api/v1/users/me", {
-      method: "PATCH",
-      headers: { "Content-Type": "application/json" },
-      body: JSON.stringify({ locale: next }),
-    }).catch(() => {});
+    try {
+      const res = await fetch("/api/v1/users/me", {
+        method: "PATCH",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ locale: next }),
+      });
+      if (!res.ok) throw new Error("Failed to save locale");
+      return true;
+    } catch {
+      // Revert on failure so the UI doesn't claim a language that never saved.
+      setLocaleState(previous);
+      setCurrentMessages(previousMessages);
+      localStorage.setItem(STORAGE_KEY, previous);
+      return false;
+    }
   }
 
   return (