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/app/(app)/settings/layout.tsx

@@ -13,7 +13,7 @@ export default async function SettingsLayout({ children }: { children: React.Rea
         <h1 className="text-3xl font-bold tracking-tight">{m.settings.title}</h1>
         <p className="text-muted-foreground mt-1">{m.settings.subtitle}</p>
       </div>
-      <div className="flex gap-8 items-start">
+      <div className="flex flex-col md:flex-row gap-4 md:gap-8 md:items-start">
         <SettingsSidebar />
         <main className="flex-1 min-w-0 space-y-6">{children}</main>
       </div>