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/App.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
   import { onMount } from 'svelte';
-  import { writable } from 'svelte/store';
+  import { get, writable } from 'svelte/store';
   import { apiFetch } from './lib/api';
   import type { SlabDraft, Photo, Taxonomy } from './lib/types';
   import { draft } from './lib/types';
@@ -98,7 +98,7 @@
     const current = $slab;
     try {
       if (!current) {
-        const s = captureSession.state;
+        const s = get(captureSession.state);
         const results = s.capture.k === 'measure' ? s.capture.results : null;
         const d = results
           ? draft({

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;