🐛 Fix removeChild crash on portal-on-document* unmount

The previous implementation passed document.body directly as the
React portal containerInfo. During unmount, React's commit phase
(commitUnmountFiberChildrenRecursively, case 4) sets the current
container to containerInfo and then calls container.removeChild()
for every DOM node inside the portal tree.

When two concurrent state updates are processed — e.g. navigating
away from a dashboard section while a file-menu portal is open —
React could attempt document.body.removeChild(node) twice for the
same node, the second time throwing:

  NotFoundError: Failed to execute 'removeChild' on 'Node':
  The node to be removed is not a child of this node.

The fix allocates a dedicated <div> container per portal instance
via mf/use-memo. The container is appended to body on mount and
removed in the effect cleanup. React then owns an exclusive
containerInfo and its unmount path never races with another
portal or the modal container (which also targets document.body).

Signed-off-by: Andrey Antukh <niwi@niwi.nz>

frontend/src/app/main/ui/components/portal.cljs

@@ -11,6 +11,11 @@
 
 (mf/defc portal-on-document*
   [{:keys [children]}]
-  (mf/portal
-   (mf/html [:* children])
-   (dom/get-body)))
+  (let [container (mf/use-memo #(dom/create-element "div"))]
+    (mf/with-effect []
+      (let [body (dom/get-body)]
+        (dom/append-child! body container)
+        #(dom/remove-child! body container)))
+    (mf/portal
+     (mf/html [:* children])
+     container)))