feat(slabs): S2 CRUD + optimistic concurrency + soft delete + slab list UI

- backend/slabs.py: create (calibrated, client-derived stored as sent), list (excl
  soft-deleted, most recent), get, update (single compare-and-set on server_rev,
  409 revision_conflict / 409 duplicate_sku / 422 stale_field, status server-derived),
  delete (soft, published rejected). Explicit column allowlists, parameterized SQL,
  JSON array fields, commits.
- backend/app.py: wire slabs router.
- frontend: src/api/slabs.ts client + create/slabs/SlabList.svelte + dev toggle.
- tests: 9 slabs tests (create/dup/list/get/update/rev-conflict/dup-sku/soft-delete/
  published-delete) with full taxonomy seeding.

Verified: pytest 23 passed; check_requirements + check_coverage exit 0; npm build ok.

frontend/src/App.svelte

@@ -10,6 +10,7 @@ import MaskPanel from './capture/MaskPanel.svelte';
 import AxisPanel from './capture/AxisPanel.svelte';
 import MeasurePanel from './capture/MeasurePanel.svelte';
 import Settings from './create/settings/Settings.svelte';
+import SlabList from './create/slabs/SlabList.svelte';
 
   const gate = typeof window !== 'undefined' ? supported() : 'window unavailable';
   const session: CaptureSession = createCaptureSession();
@@ -18,6 +19,7 @@ import Settings from './create/settings/Settings.svelte';
   let busy = false;
   let pngUrl: string | null = null;
   let showSettings = false;
+  let showSlabs = false;
 
   const unsubCap = session.capture.subscribe((c) => {
     cap = c;
@@ -90,10 +92,16 @@ import Settings from './create/settings/Settings.svelte';
         {pngUrl}
       />
     {/if}
-    <button on:click={() => (showSettings = !showSettings)} style="margin-top:1rem">Settings (dev)</button>
+    <div style="margin-top:1rem;display:flex;gap:8px">
+      <button on:click={() => (showSettings = !showSettings)}>Settings (dev)</button>
+      <button on:click={() => (showSlabs = !showSlabs)}>Slabs (dev)</button>
+    </div>
     {#if showSettings}
       <Settings />
     {/if}
+    {#if showSlabs}
+      <SlabList />
+    {/if}
   </main>
 {/if}