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/homework/HomeworkScreen.tsx

@@ -20,6 +20,7 @@ import { useI18n } from "@/lib/i18n";
 import type { HomeworkMode, HomeworkRecord } from "@/lib/schemas";
 import { useNotesIntake } from "@/lib/use-notes-intake";
 import { clearWizardDraft } from "@/lib/wizard-draft";
+import { wizardBusy } from "@/lib/wizard-busy";
 
 import { HomeworkCheckResult } from "./HomeworkCheckResult";
 import { HomeworkGuide } from "./HomeworkGuide";
@@ -65,6 +66,17 @@ export function HomeworkScreen({ initialRecordId }: { initialRecordId?: string }
     if (record) setView({ kind: "record", record });
   }, [initialRecordId]);
 
+  /* A loading run locks the mode switch (see WizardModeSwitch). */
+  const viewKind = view.kind;
+  useEffect(() => {
+    wizardBusy.set(viewKind === "loading");
+  }, [viewKind]);
+  useEffect(() => () => wizardBusy.set(false), []);
+
+  /* Leaving the page mid-run stops the request instead of letting it finish
+     invisibly and save a result the user never sees. */
+  useEffect(() => () => abortRef.current?.abort(), []);
+
   /** Everything the API needs, built from the current input. */
   const buildPayload = (modeValue: HomeworkMode) => {
     const readyDocuments = intake.documents.filter((entry) => entry.status === "ready" && entry.extracted);