Let the native clipboard through where the canvas has nothing to copy

A bound chord used to be `preventDefault`ed before its handler ran, so the
canvas took every ⌘C and ⌘V whether or not the node clipboard would act —
which is what blocked copying selected text out of the chrome floating over
the canvas. A binding may now return `false` to decline the key; ⌘C does so
when there is a text selection or no node selected, ⌘V when nothing of ours
is stored.

frontend/src/components/Flow/FlowEditor.tsx

@@ -1061,8 +1061,21 @@ function FlowEditorInner({
     {
       "mod+z": () => step(true),
       "mod+shift+z": () => step(false),
-      "mod+c": () => void copyNodes(),
-      "mod+v": pasteNodes,
+      // Both decline the key unless the node clipboard has something to do
+      // with it, so the native copy and paste still work on the canvas. A
+      // text selection wins outright: that is what the user asked to copy.
+      "mod+c": () => {
+        const selection = document.getSelection()
+        if (selection && !selection.isCollapsed) return false
+        if (!canvasNodes.some((node) => node.selected) && !selectedId) {
+          return false
+        }
+        void copyNodes()
+      },
+      "mod+v": () => {
+        if (!safeStorage.get(CLIPBOARD_KEY)) return false
+        pasteNodes()
+      },
       // ⌘P, not ⌘K: the sidebar's global search owns that everywhere, and this
       // palette is the canvas's own, narrower thing.
       "mod+p": () => setPaletteOpen((open) => !open),