fix(core): add rebuild-live-log class to live-log container div

The CSS for the rebuild-queue live-log panel targets .rebuild-live-log
(border, border-radius, margin-top, background) but the HTML element only
had id="rebuild-live-log" — no class. As a result the panel box styles
never applied and the live log rendered unstyled (no border, no background,
no visual separation from the queue rows).

Fix: add class="rebuild-live-log" to the element so the CSS selector
matches.

frontend/packages/dashboard/src/core.html

@@ -59,7 +59,7 @@
            time). Managed by renderRebuildLiveLog in core.js, separate from
            rebuild-queue-section so the queue's row re-render never disturbs
            the open SSE stream. -->
-      <div id="rebuild-live-log" hidden></div>
+      <div id="rebuild-live-log" class="rebuild-live-log" hidden></div>
     </section>
 
     <!-- M3T4 1NPUTS: select inputs to nix flake update in /meta/. -->