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.

components/WizardModeSwitch.tsx

@@ -5,16 +5,21 @@ import Link from "next/link";
 import { SegmentedPill } from "@/components/SegmentedPill";
 import { useI18n } from "@/lib/i18n";
 import { useIsPhone } from "@/lib/use-phone";
+import { useWizardBusy } from "@/lib/wizard-busy";
 import { cn } from "@/lib/utils";
 
 /**
  * In-page switch between the two wizards (New deck ⇄ Make my homework).
  * Phones reach both through this control and the centre tab; desktop keeps
- * the rail rows as well.
+ * the rail rows as well. Hidden while either wizard is generating: leaving a
+ * loading run through it would abandon the request mid-flight.
  */
 export function WizardModeSwitch({ mode }: { mode: "deck" | "homework" }) {
   const { t } = useI18n();
   const isPhone = useIsPhone();
+  const busy = useWizardBusy();
+
+  if (busy) return null;
 
   return (
     <div className="segmented mb-5" role="group" aria-label={t("wizard.mode.aria")}>