Lock the wizard mode switch while a run is generating

Reported: during a loading screen (freezing a deck) the Study deck /
Make my homework switch stayed live, so the run could be swapped out
mid-flight; a request that finished afterwards could even save a deck
the user never saw and navigate them into it.

- New lib/wizard-busy.ts: a tiny store both wizards publish to while
  phase === 'loading' (UploadScreen) or view.kind === 'loading'
  (HomeworkScreen); WizardModeSwitch hides itself while busy.
- Both wizards abort the in-flight request on unmount, so leaving via
  the header logo or the Back gesture stops the run instead of letting
  it finish invisibly and save/navigate later.
- Deck Cancel keeps its long-standing behaviour (wizard + cancelled
  notice + Retry), matching the freeze-loading suite.

Verified: wizard suite 92 checks (mode switch hidden during deck and
homework runs, it returns after Cancel, and a delayed reply after
leaving mid-run saves nothing and navigates nowhere) plus mobile
134/134 (also de-flaked the sheet's Escape close by waiting for
detach), regression 74/74, declutter 116/116, animation 8/8, native
35/35, i18n 25/25, phase1 38/38, phase2 43/43, phase3 20/20,
freeze-loading 6/6. Lint + build clean. SW freezmo-v29.

lib/wizard-busy.ts

@@ -0,0 +1,34 @@
+"use client";
+
+/**
+ * True while either wizard is generating. The in-page mode switch reads this
+ * and hides itself, so a loading run can't be swapped out mid-flight — the
+ * loading screen owns the page until it finishes or is cancelled.
+ */
+
+import { useSyncExternalStore } from "react";
+
+let busy = false;
+const listeners = new Set<() => void>();
+
+export const wizardBusy = {
+  subscribe(listener: () => void): () => void {
+    listeners.add(listener);
+    return () => listeners.delete(listener);
+  },
+  getSnapshot(): boolean {
+    return busy;
+  },
+  getServerSnapshot(): boolean {
+    return false;
+  },
+  set(value: boolean): void {
+    if (busy === value) return;
+    busy = value;
+    listeners.forEach((listener) => listener());
+  },
+};
+
+export function useWizardBusy(): boolean {
+  return useSyncExternalStore(wizardBusy.subscribe, wizardBusy.getSnapshot, wizardBusy.getServerSnapshot);
+}