External embed: fix code-dialog modal visibility

openEmbedCodeDialog set modal.style.display = '' to show the
modal, which falls back to the CSS default `.modal { display:
none }` — so the dialog never appeared. Every other modal in the
app sets `'flex'` explicitly. Matching that pattern.

Also fixes the symptom "Embed button doesn't work / doesn't
copy anything" — the dialog was always present in the DOM but
invisible, so the copy buttons inside it couldn't be reached.

src/index.html

@@ -21782,7 +21782,7 @@
       const modal = $('embedCodeModal');
       if (!modal) return;
       modal._row = row;
-      modal.style.display = '';
+      modal.style.display = 'flex';
       modal.setAttribute('aria-hidden', 'false');
       bringModalToFront?.(modal);
       // Reset to sensible defaults each open.