feat(09-03): apply MD3 button styles and elevation across all wizard steps

- BackendCard: rounded-lg -> rounded-xl, added shadow/shadow-md elevation
- OutputBlock: pre rounded -> rounded-xl + shadow-sm; Copy/Download use MD3_BTN_OUTLINED small variant
- BackendSelectionStep: added explicit Next submit button with MD3_BTN_FILLED
- RemoteConfigStep: Back uses MD3_BTN_OUTLINED, Next uses MD3_BTN_FILLED
- DeploymentStep: Back uses MD3_BTN_OUTLINED, Next/Review uses MD3_BTN_FILLED
- ReviewStep: Back uses MD3_BTN_OUTLINED, Download All (ZIP) uses MD3_BTN_FILLED
- All 179 tests pass (zero regressions)

src/components/wizard/ReviewStep.tsx

@@ -3,6 +3,7 @@
 // Covers CONF-02, CONF-03, DOWN-01–DOWN-06, SECU-01, SECU-02.
 import { useMemo, useState } from 'react';
 import { useWizard } from '../../store/context';
+import { MD3_BTN_FILLED, MD3_BTN_OUTLINED } from '../../styles/md3-buttons';
 import {
   buildRcloneConf,
   buildIntuneInstall,
@@ -128,7 +129,7 @@ export function ReviewStep() {
         <button
           type="button"
           onClick={() => dispatch({ type: 'SET_STEP', payload: 2 })}
-          className="px-4 py-2 text-sm border border-outline rounded-md hover:bg-surface"
+          className={MD3_BTN_OUTLINED}
         >
           Back
         </button>
@@ -139,7 +140,7 @@ export function ReviewStep() {
         type="button"
         disabled={!acknowledged}
         onClick={handleDownloadZip}
-        className="w-full py-2 px-4 bg-primary text-on-primary rounded font-medium disabled:opacity-40 disabled:cursor-not-allowed hover:bg-primary"
+        className={`w-full mt-4 ${MD3_BTN_FILLED}`}
       >
         Download All (ZIP)
       </button>