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.

README.md

@@ -52,7 +52,10 @@ no database, no data leaving your machine except the requests you send to Ollama
   real summary — thumbnails/chips, files, difficulty, activities and folder, each with an
   *Edit* link back to its step. A flat Back/Cancel + Next/Freeze bar sits at the bottom above
   the safe area (the tab bar hides inside the wizard), and it is portalled to `<body>` so the
-  screen entrance animation can never displace it. The desktop page is unchanged.
+  screen entrance animation can never displace it. While a run is generating, the wizard's
+  **mode switch hides** — a loading screen can't be swapped to the other wizard mid-flight —
+  and leaving the page anyway aborts the request instead of saving a deck in the background.
+  The desktop page is unchanged.
 - **Native-feel touches (phones)**: the Android Back gesture closes the topmost layer first
   (sheet, actions sheet, then one wizard step); deleting a deck shows an *Undo*
   toast instead of a confirm; the deck menu can share a deck as JSON through the system share
@@ -1034,9 +1037,12 @@ Things I chose while building, with the reasoning:
     Drawer-only destinations were re-homed: Progress and Exam became tabs, the New deck screen got
     a **Study deck ⇄ Make my homework** segmented switch (also on `/homework`), and the library's
     phone row gained a **⋯** sheet with Import JSON + New folder (folder chips still appear only
-    once a folder exists). Verified by the mobile suite (134 checks: no hamburger/drawer, tab order,
-    the label matrix, per-tab routing, deep links, Back from every tab and the ⋯ sheet) plus the
-    other ten standing suites.
+    once a folder exists). The mode switch is hidden while either wizard is generating
+    (`lib/wizard-busy.ts`), and leaving a wizard mid-run aborts the request so a stray generation
+    can never save a deck or yank the user into the study feed. Verified by the mobile suite
+    (134 checks: no hamburger/drawer, tab order, the label matrix, per-tab routing, deep links,
+    Back from every tab and the ⋯ sheet) plus the wizard suite (92 checks, including the
+    loading-lock and abort-on-leave cases) and the other standing suites.
 
 36. **The mobile declutter pass reshapes, it doesn't fork.** Phones (≤639px) get a 3-step deck
     flow and a 2-step homework flow that share the exact state and handlers with the desktop