Rename remaining visible "Nostr OS" strings to LAYER.systems

Covers the About window heading, the mobile shell header and the app
icon's aria-label, so the visible branding matches the page title.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014YtQoCzkP7Bo8nruhxojPi

public/icon.svg

@@ -1,4 +1,4 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" role="img" aria-label="Nostr OS">
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 512 512" role="img" aria-label="LAYER.systems">
   <rect width="512" height="512" rx="112" fill="#8b5cf6"/>
   <rect x="104" y="128" width="304" height="256" rx="28" fill="#ffffff"/>
   <rect x="104" y="128" width="304" height="52" rx="28" fill="#ede9fe"/>

src/apps/about/index.tsx

@@ -27,7 +27,7 @@ export default function AboutApp({ setTitle }: AppProps) {
             <span className="inline-flex size-11 items-center justify-center rounded-xl border border-border bg-background shadow-sm">
               <Zap className="size-5 text-primary" aria-hidden />
             </span>
-            <h1 className="text-xl font-semibold tracking-tight">Nostr OS</h1>
+            <h1 className="text-xl font-semibold tracking-tight">LAYER.systems</h1>
             <p className="text-sm leading-relaxed text-muted-foreground">
               A Nostr client shaped like a desktop. Every part of it is an app in a window you
               can move, resize, stack and keep open side by side — reading a thread does not

src/components/os/MobileAppShell.tsx

@@ -46,7 +46,7 @@ export function MobileAppShell() {
         ) : (
           <span className="flex items-center gap-1.5 text-sm font-semibold">
             <Zap className="size-4 text-primary" aria-hidden />
-            Nostr OS
+            LAYER.systems
           </span>
         )}