Fix app shell layout with stable top bar and dynamic nav offset.

Unified chrome keeps the home logo top-left, measures bottom nav height
for scroll clearance, and adds responsive content widths for mobile/desktop.

components/ui/toast-provider.tsx

@@ -45,8 +45,8 @@ export function ToastProvider({ children }: { children: ReactNode }) {
       {children}
       <div
         aria-live="polite"
-        className="pointer-events-none fixed right-4 z-50 flex w-[min(100vw-2rem,20rem)] flex-col gap-2"
-        style={{ bottom: "calc(var(--bottom-nav-height) + 0.75rem)" }}
+        className="pointer-events-none fixed right-4 z-[60] flex w-[min(100vw-2rem,20rem)] flex-col gap-2"
+        style={{ bottom: "calc(var(--app-nav-offset, var(--bottom-nav-height)) + 0.75rem)" }}
       >
         {messages.map((message) => (
           <div