Chat mode, phase 2: local history, answer actions, motion polish

- Conversations in a versioned IndexedDB store (freezmo-chat v1, zod
  validated with the same schema backup/sync use, capped at 100 x 300).
  Settled turns persist (question always; answer when produced, partial
  stops included); ?c=<id> keeps reloads on the conversation via
  history.replaceState (no router round-trip mid-chat).
- History UI: desktop column, phone sheet (spring, backdrop, Escape),
  auto titles from the first question, inline rename, delete with an
  Undo toast, diacritic-folding search over titles and message text.
  Scope and model are remembered per conversation; history is readable
  offline.
- Assistant actions: Turn into flashcards (New deck with the answer as
  notes, landing on the options step - deck seed gained step) and Save
  to notes (appends to the single scoped deck, trimmed at 40k).
- Chats travel in the backup export (preview counts them) and in the
  encrypted sync payload; still never the key.
- Motion from lib/motion.ts: message fade-up, sheet springs for the
  model/context/history sheets, 150ms composer height transition, all
  skipped under reduced motion.
- Tests: chat suite 63 -> 89 checks (history, reload, rename/search/
  delete/undo, actions, export, two-device sync round-trip, offline,
  desktop panel). Full set: 12/12 suites, 513 checks. SW -> freezmo-v33.

Also: tests/model-compare.mjs benchmark (diagnostic) for deciding the
default text model on numbers; defaults unchanged.

components/UploadScreen.tsx

@@ -328,6 +328,7 @@ export function UploadScreen() {
     setText(seed.text);
     if (seed.difficulty) setDifficulty(seed.difficulty);
     if (seed.count) setCount(seed.count);
+    if (seed.step && seed.step > 1) setStep(seed.step);
   }, []);
 
   /* Persist the draft while the user works (debounced, best-effort). */