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 <[email protected]>

locales/el-GR/opencadstudio.ftl

@@ -167,6 +167,7 @@ clipboard =
     .paste-to-original-coordinates = Επικόλληση στις αρχικές συντεταγμένες
 
 viewport =
+    .preparing-layout = Προετοιμασία διάταξης…
     .split-vertical = Κατακόρυφος διαχωρισμός θύρας προβολής
     .split-horizontal = Οριζόντιος διαχωρισμός θύρας προβολής
     .move = Μετακίνηση θύρας προβολής

locales/en-US/opencadstudio.ftl

@@ -167,6 +167,7 @@ clipboard =
     .paste-to-original-coordinates = Paste to Original Coordinates
 
 viewport =
+    .preparing-layout = Preparing layout…
     .split-vertical = Split Viewport Vertically
     .split-horizontal = Split Viewport Horizontally
     .move = Move Viewport