feat: draw a notice for one frame before rebuilding a layout

Switching to a Layout stops the main thread long enough that the
compositor decides the application has stopped responding and offers to
kill it. The switch itself is not the problem — `layout-switch` reports
about 10 ms — it is the frame after it, which rebuilds the whole wire set.

That frame now has a cheap one in front of it. `layout_settling` is set
when the switch takes, and while it holds, the viewport is replaced by a
centered "Preparing layout…" on the canvas background. No shader widget
means no scene primitive, so `prepare` has nothing to do and the frame
costs nothing. A `window::frames` subscription clears the flag, which is
the guarantee this needs: that callback runs once a frame has actually
been presented, so the notice is on screen and the compositor has been
answered before the expensive frame starts.

The deferral is of the *frame*, not the switch. Two callers depend on the
switch having completed when it returns — one calls `fit_all` immediately
after, another compares `current_layout` — so deferring the switch itself
would have broken both.

This restarts the compositor's timer immediately before the slow frame and
replaces a stale window with something that says what is happening. It
does not make the slow frame fast: if the rebuild still exceeds the
compositor's timeout the dialog can return, and the durable fix is the
2 s `memo_miss` behind it, which is a separate change.

Only `en-US` is validated against at build time; Greek is filled in too
since it is in use here, and the rest fall back until translated.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/app/update/mod.rs

@@ -3045,6 +3045,12 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::LayoutSettled => {
+                // The notice frame has been presented, so the compositor has
+                // had its answer. Let the next frame build the real thing.
+                self.layout_settling = false;
+                Task::none()
+            }
             Message::GripDwellTick => {
                 let i = self.active_tab;
                 // Reuse the move-time logic — `p` is the last cursor