Persist the studio session across reloads

The working photo and every knob the workspace holds now survive a reload,
for guests as much as for signed-in users:

- engine/session.ts: the knobs go to localStorage (rc.studio.v1) as small
  JSON; the photo goes to IndexedDB, because a 12MP JPEG does not fit in
  localStorage. Both fail soft (private mode, quota) — the studio still works,
  it just forgets.
- App.tsx: state seeds from the stored snapshot synchronously, so the first
  paint already holds the user's settings; the boot effect pulls the photo
  back and adopts it with keepGeo, so the restored params are not clobbered
  by the photo's own EXIF.

Recipes a guest creates with SAVE RECIPE stay session-only, as asked — they
are still gone on reload (create-test asserts it).

docker/README.md

@@ -35,6 +35,7 @@ backend/                Fastify + better-sqlite3 API, own Dockerfile
 frontend/               Vite + React + CanvasKit SPA, own Dockerfile + nginx.conf
   shared/               vendored copies of the app's types + utils (see below)
   src/engine/           skiaShim.ts (CanvasKit) + exportEngine.ts (render pipeline)
+                        + session.ts (localStorage/IndexedDB studio persistence)
 ```
 
 ### Vendored files