fix(fe): subscribe to capture-session store in components (was reading the store object)

`session.state` is a Svelte store; `$: state = session.state` bound the store
object (not its value), so state.capture was undefined and cap.k threw on
mount. Bind a local const to the store and use the $ prefix ($stateStore),
and use get() in App.onPhotosReady. Verified in Chromium: hero -> batch ->
sample uploads show thumbs.

frontend/src/capture/PhotosReady.svelte

@@ -7,7 +7,8 @@
   export let session: CaptureSession;
   const dispatch = createEventDispatcher<{ continue: void }>();
 
-  $: state = session.state;
+  const stateStore = session.state;
+  $: state = $stateStore;
   $: items = session.lineup();
   $: measure = state.capture.k === 'measure' ? state.capture : null;
   $: results = measure?.results ?? null;

frontend/src/capture/SessionCapture.svelte

@@ -6,7 +6,8 @@
   export let session: CaptureSession;
   const dispatch = createEventDispatcher<{ 'photos-ready': void }>();
 
-  $: state = session.state;
+  const stateStore = session.state;
+  $: state = $stateStore;
   $: cap = state.capture;
   $: kind = cap.k;