fix(maestro): keep the keyboard in the list while previewing

Two mounts were taking focus out of the sidebar mid-walk: the render that
put the multi-view on screen before the preview applied, and the render
that restores the previous pane when a preview ends. Both now mount
without focusing, so j/k keeps walking. Also marks the preview with a
badge on the canvas and drops the close button from its tab.

web/src/styles.css

@@ -2890,6 +2890,21 @@ input.leaf-rename:focus {
   border-style: dashed;
   font-style: italic;
 }
+.mv.mv-previewing .mv-canvas::before {
+  content: "preview";
+  position: absolute;
+  top: var(--sp-2);
+  right: var(--sp-3);
+  z-index: var(--z-sticky);
+  padding: 1px var(--sp-2);
+  border-radius: var(--r-1);
+  font: 500 var(--fs-11) / 1.5 var(--font-mono);
+  letter-spacing: 0.04em;
+  color: var(--accent-ink);
+  background: var(--accent);
+  opacity: 0.85;
+  pointer-events: none;
+}
 .mv.mv-previewing .mv-canvas::after {
   content: "";
   position: absolute;