frontend: runtime backend URL (LAN), voice dictation, version + update UI

Resolve the backend URL from the current host at runtime (lib/backend-url.ts)
so one build works on localhost and any clinic LAN IP without a rebuild; used
by the API client, auth client, and socket. Wire the AI-chat mic to the Web
Speech API with graceful fallback. Add a Settings "About & updates" panel
(current/latest version, update command, shareable network address) and an
optional dismissible update banner.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/app/(app)/layout.tsx

@@ -3,6 +3,7 @@ import { CommandPaletteProvider } from "@/components/command-palette";
 import { DashboardSidebar } from "@/components/sidebar-02/app-sidebar";
 import { MobileSidebarTrigger } from "@/components/sidebar-02/mobile-sidebar-trigger";
 import { SidebarProvider } from "@/components/ui/sidebar";
+import { UpdateBanner } from "@/components/update-banner";
 
 export default function AppLayout({
   children,
@@ -17,6 +18,7 @@ export default function AppLayout({
             <DashboardSidebar />
             <MobileSidebarTrigger />
             {children}
+            <UpdateBanner />
           </div>
         </SidebarProvider>
       </CommandPaletteProvider>