Canvas gives no illustrative feedback/loading state while AI workflow generation is running

Epic: #4848 (AI-first UX)
Related: #4868 (Build with AI)

Problem

After clicking "Build it" in the Build with AI flow, generation can take
30+ seconds. The canvas itself shows nothing during that window, and the
only feedback anywhere is the chat window's typing dots — which aren't very
illustrative of what's actually happening. A first-time user watching an
empty canvas with just some dots elsewhere on screen could easily think the
app has frozen rather than that a workflow is being built.

Direction

Put something rough and ready on the canvas while generation is in
progress — e.g. a placeholder workflow shape — just enough to signal "this
is building" rather than "this is broken." It doesn't need to be polished;
a nicer, more illustrative loading UX (richer incremental feedback from the
assistant, etc.) can come later as its own piece of work.

Scope

• Show a rough placeholder on the canvas from the moment "Build it" is
  clicked until the generated workflow is ready to render.
• Keep it intentionally simple — a stopgap, not the final loading UX.
• Leave room for a follow-up to improve this once richer, incremental
  assistant feedback is available.

Start at the Build with AI flow and trace the handler for the “Build it” action through the period before the generated workflow renders on the canvas. Show a simple placeholder during generation, remove it when the workflow is ready, and verify that the empty canvas is visibly distinct while building.