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/App.svelte

@@ -98,7 +98,7 @@
     const current = $slab;
     try {
       if (!current) {
-        const s = $captureSession.state;
+        const s = captureSession.state;
         const results = s.capture.k === 'measure' ? s.capture.results : null;
         const d = results
           ? draft({

frontend/src/capture/PhotosReady.svelte

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

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;