feat: Improve UI layout and navigation

- Increase logo size (48x48 desktop, 56x56 mobile) for better visibility
- Add logo as favicon
- Add logo to mobile header
- Move user menu to navigation bars (sidebar on desktop, bottom bar on mobile)
- Fix desktop chat layout - container structure prevents voice controls cutoff
- Fix mobile bottom bar - use icon-only ActionIcons instead of truncated text buttons
- Hide Create Node/New Conversation buttons on mobile to save header space
- Make fixed header and voice controls work properly with containers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

hooks/useAppMachine.ts

@@ -0,0 +1,24 @@
+/**
+ * useAppMachine Hook
+ *
+ * Provides access to the app-level state machine from any component.
+ * Must be used within an AppStateMachineProvider.
+ */
+
+import { createContext, useContext } from 'react';
+import type { ActorRefFrom } from 'xstate';
+import { appMachine } from '@/lib/app-machine';
+
+type AppMachineActor = ActorRefFrom<typeof appMachine>;
+
+export const AppMachineContext = createContext<AppMachineActor | null>(null);
+
+export function useAppMachine() {
+  const actor = useContext(AppMachineContext);
+
+  if (!actor) {
+    throw new Error('useAppMachine must be used within AppStateMachineProvider');
+  }
+
+  return actor;
+}