fix: keep embedded session controls visible and handle empty memory

Cap the embedded shell at its portal container height so steering and stop controls remain accessible. Skip vector retrieval for an empty authoritative Memory archive and compute SQL-rule embeddings lazily.

Validated with 54 Memory tests, 90 frontend tests, five browser scenarios, frontend and Docker builds, and a read-only comparison against the real empty Memory archive.

frontend/src/index.css

@@ -222,6 +222,8 @@ body.thot-full-document { margin: 0; }
     background: oklch(var(--background));
     color-scheme: light;
   }
+  /* Embedded hosts can reserve space for their own header/footer around #root. */
+  .thot-host--embedded { max-height: 100%; }
   .thot-host[data-theme="dark"] { color-scheme: dark; }
   .thot-host--full { --thot-shell-gutter: max(20px, 1.5rem); }
   .thot-host--full > .thot-context-layout {