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/components/settings/settings-form.tsx

@@ -118,7 +118,15 @@ export function SettingsForm({ user }: { user: UserProps }) {
       <section className="rounded-xl border p-6 space-y-4">
         <h2 className="font-semibold text-lg">{t("language")}</h2>
         <p className="text-sm text-muted-foreground">{t("languageDescription")}</p>
-        <Select defaultValue={user.locale} onValueChange={(v) => setLocale(v as Locale)}>
+        <Select
+          defaultValue={user.locale}
+          onValueChange={(v) => {
+            void setLocale(v as Locale).then((ok) => {
+              if (ok) toast.success(t_common("saved"));
+              else toast.error(t_common("saveFailed"));
+            });
+          }}
+        >
           <SelectTrigger className="w-48">
             <SelectValue />
           </SelectTrigger>