fix: defer the toolbar tick past Svelte's flush

Tearing down the focused pane's DOM on a layout switch fires blur
synchronously inside Svelte's commit; TipTap answers with a focus-meta
transaction, and onTransaction's tick write threw state_unsafe_mutation
there — aborting the handler before the other panes got the new state.
The tick now bumps in a microtask, after the flush, before paint.

src/lib/components/Editor.svelte

@@ -1063,7 +1063,9 @@ import { EMPTY_PAGE_DECOR, type PageDecor } from '../storage/pageDecor';
         },
       },
       onTransaction: ({ editor: e, transaction }) => {
-        tick++;
+        // Deferred: a blur tr arrives synchronously when a pane-layout switch tears
+        // down the focused DOM — inside Svelte's flush, where a $state write throws.
+        queueMicrotask(() => tick++);
         // The other pane shows the same state, decorations included.
         for (const view of paneViews) view?.updateState(e.state);
         // Mirror this transaction into the labelled undo/redo log for the toolbar's