feat(08-01): implement ThemeToggle component and wire into App.tsx

- Add ThemeToggle segmented control (Light/Dark/System) with DOM class toggle
- Use getStored() lazy init and applyTheme() for side-effect-free render
- Stub localStorage and matchMedia in tests for Node v25 compatibility
- Wire ThemeToggle into App.tsx header flex row next to h1
- Change App outer div to bg-surface, h1 to text-on-surface
- All 7 ThemeToggle unit tests passing; 166 total tests green

src/App.tsx

@@ -4,6 +4,7 @@
 
 import { useWizard } from './store/context';
 import { WizardProvider } from './store/context';
+import { ThemeToggle } from './components/ui/ThemeToggle';
 import { StepIndicator } from './components/wizard/StepIndicator';
 import { BackendSelectionStep } from './components/wizard/BackendSelectionStep';
 import { RemoteConfigStep } from './components/wizard/RemoteConfigStep';
@@ -26,9 +27,12 @@ function WizardShell() {
   const CurrentStep = steps[stepIndex];
 
   return (
-    <div className="min-h-screen bg-gray-50 flex flex-col items-center py-12 px-4">
+    <div className="min-h-screen bg-surface flex flex-col items-center py-12 px-4">
       <div className="w-full max-w-2xl">
-        <h1 className="text-3xl font-bold text-gray-900 mb-8 text-center">Ready2Blob</h1>
+        <div className="flex items-center justify-between mb-8">
+          <h1 className="text-3xl font-bold text-on-surface">Ready2Blob</h1>
+          <ThemeToggle />
+        </div>
         <StepIndicator />
         <div className="mt-8">
           {CurrentStep}