fix(fe): hero 'Start capture (session)' enters the M2 session, not the chooser

The hero button called flow.set('capture') which renders the path-chooser
branch, so it stayed on the chooser and 'Choose photos' never appeared.
Point it at 'captureSession' so the hero action lands directly in the batch
screen. Verified in Chromium: hero button -> batch screen -> sample uploads
show thumbs.

frontend/src/App.svelte

@@ -244,7 +244,7 @@
       <p>Capture • Measure • Review • Publish — real store API, SLAB-UAT-* publishes as draft</p>
       <div class="actions">
         <button on:click={startFresh} class="primary">Start new slab</button>
-        <button on:click={() => flow.set('capture')}>Start capture (session)</button>
+        <button on:click={() => flow.set('captureSession')}>Start capture (session)</button>
         {#if $slab}
           <button on:click={resume}>Resume draft</button>
         {/if}