feat(publish): S6 Woo create sequence + UAT draft + dup/stale/partial failure + Publish UI

- backend/publish.py: POST /slabs/{id}/publish. Sequence (PUBLISH-5): validate ready
  set -> SKU lookup (409 on dup, PUBLISH-15) -> refresh taxonomy if >1h (SETTINGS-60)
  -> resolve cats/tags/5 attributes -> upload processed PNGs -> POST /products ->
  finalize (set woo_product_id, purge photos, PUBLISH-25). SLAB-UAT-* always draft
  (PUBLISH-10). Failure: mark failed, retain photos, sync_log, plain recoverable/
  unrecoverable message (PUBLISH-30/36/37/38). Partial (media ok, POST failed):
  orphan media logged + manual cleanup note (PUBLISH-31). Payload: only non-null,
  slug = name+sku, sold_individually true (PUBLISH-55/60).
- frontend: create/publish/Publish.svelte (slab id, publish, result + recoverable/
  orphan tags) + dev toggle.
- tests: 5 publish tests (success+purge, UAT draft, not-ready 422, dup 409,
  partial-media 502). Woo transport mocked.

Verified: pytest 48 passed; gates exit 0; npm build ok.

frontend/src/App.svelte

@@ -13,6 +13,7 @@ import Settings from './create/settings/Settings.svelte';
 import SlabList from './create/slabs/SlabList.svelte';
 import PhotoAttach from './create/photos/PhotoAttach.svelte';
 import Review from './create/review/Review.svelte';
+import Publish from './create/publish/Publish.svelte';
 
   const gate = typeof window !== 'undefined' ? supported() : 'window unavailable';
   const session: CaptureSession = createCaptureSession();
@@ -24,6 +25,7 @@ import Review from './create/review/Review.svelte';
   let showSlabs = false;
   let showPhotos = false;
   let showReview = false;
+  let showPublish = false;
 
   const unsubCap = session.capture.subscribe((c) => {
     cap = c;
@@ -101,6 +103,7 @@ import Review from './create/review/Review.svelte';
       <button on:click={() => (showSlabs = !showSlabs)}>Slabs (dev)</button>
       <button on:click={() => (showPhotos = !showPhotos)}>Photos (dev)</button>
       <button on:click={() => (showReview = !showReview)}>Review (dev)</button>
+      <button on:click={() => (showPublish = !showPublish)}>Publish (dev)</button>
     </div>
     {#if showSettings}
       <Settings />
@@ -114,6 +117,9 @@ import Review from './create/review/Review.svelte';
     {#if showReview}
       <Review />
     {/if}
+    {#if showPublish}
+      <Publish />
+    {/if}
   </main>
 {/if}