fix(fe): read capture-session store directly (no $ prefix on the object)

`createCaptureSession()` returns {state: Store, ...}; Svelte's $ prefix
calls .subscribe on the object, not the store, so $captureSession.state
threw 'subscribe is not a function' on App mount → black page. Reference the
store directly (auto-subscribes): captureSession.state / session.state /
session.lineup(). Verified: tsc clean, build green, 150/150 tests, and the
live app renders in Chromium.

frontend/src/capture/SessionCapture.svelte

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