feat(11-01): mobile-responsive layout for wizard steps

- BackendSelectionStep: responsive grid (1-col mobile, 2-col desktop) for cards; button row stacks vertically on mobile
- BackendCard: add w-full to fill grid cell on all viewports
- RemoteConfigStep/DeploymentStep/ReviewStep: flex-col sm:flex-row button rows; w-full sm:w-auto on Back/Next buttons
- StepIndicator: add hidden sm:block to all three label spans (completed, active, future)

src/components/wizard/ReviewStep.tsx

@@ -131,11 +131,11 @@ export function ReviewStep() {
       )}
 
       {/* TECH-02: Back button — dispatches SET_STEP(2) */}
-      <div className="flex gap-3 mt-6">
+      <div className="flex flex-col sm:flex-row gap-3 mt-6">
         <button
           type="button"
           onClick={() => dispatch({ type: 'SET_STEP', payload: 2 })}
-          className={MD3_BTN_OUTLINED}
+          className={`${MD3_BTN_OUTLINED} w-full sm:w-auto`}
         >
           Back
         </button>