fix(m3): uuid secure-context fallback + middle-tier SKU business rules

- frontend: uuid() helper (crypto.randomUUID -> getRandomValues) for plain-HTTP LAN UAT
- slabs.py: empty SKU at create -> NULL (no-SKU-yet is transient, filled in Review)
- review.py: ready-gate enforces REVIEW-45 (SKU 3-24 alnum+hyphen) in the middle tier
- DB CHECK stays as backstop only (referential/structural), per layering principle

frontend/src/browser.ts

@@ -24,3 +24,25 @@ export function supported(): string | null {
   if (!isIOS && !isAndroid && (isChrome || isEdge || isFirefox)) return null;
   return 'unsupported browser';
 }
+
+// v4 UUID. crypto.randomUUID() is only guaranteed on secure contexts
+// (HTTPS/localhost); the UAT host is plain HTTP on a LAN IP, where it is
+// undefined. Fall back to crypto.getRandomValues (available on HTTP) so the
+// client-generated ids (APP-20) work in both contexts.
+export function uuid(): string {
+  const c = globalThis.crypto;
+  if (c && typeof c.randomUUID === 'function') return c.randomUUID();
+  if (c && typeof c.getRandomValues === 'function') {
+    const b = new Uint8Array(16);
+    c.getRandomValues(b);
+    b[6] = (b[6] & 0x0f) | 0x40; // version 4
+    b[8] = (b[8] & 0x3f) | 0x80; // variant 10
+    const h = Array.from(b, (x) => x.toString(16).padStart(2, '0')).join('');
+    return `${h.slice(0, 8)}-${h.slice(8, 12)}-${h.slice(12, 16)}-${h.slice(16, 20)}-${h.slice(20)}`;
+  }
+  return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, (ch) => {
+    const r = (Math.random() * 16) | 0;
+    const v = ch === 'x' ? r : (r & 0x3) | 0x8;
+    return v.toString(16);
+  });
+}