scope windows: Fit X axis / Fit all in the menu, and a window that cannot paint says so

The scope menu carried a y fit only: with one shared time span, the x fit *is* the main window's
fit, and the next frame mirrors the result back -- so `Fit X axis` and `Fit all` now sit in the
menu beside the y entries and reset the shared range.

And a blank plot becomes a sentence: a scope window whose GL context failed, or whose area has
painted no frame two seconds after opening, writes the reason into its own title and to stderr
(`[scope] window 'plot 1' is not drawing: GL context failed: ...`). A blank rectangle is
indistinguishable from a failed context or an area that never got a frame, and the report "the
plot is not there, no x axis" is exactly what this answers: if the title stays unchanged, the
area is painting and the question moves to the compositor presenting the second window.

src/ui_app.hpp

@@ -233,6 +233,10 @@ struct App {
     double pointer_x = 0.0;
     double pointer_y = 0.0;
     bool pointer_valid = false;
+    /// Frames this window's area was watched for its first paint: a window whose GL context failed
+    /// or that never paints says so in its own title (and on stderr) instead of staying a blank
+    /// rectangle nobody can diagnose.
+    int paint_checks = 0;
     /// The area's initial plot, owned here: `PlotArea::create()` wants a `Plot&` that
     /// outlives the widget, and sharing the main window's placeholder would hand two areas
     /// one plot the moment the figure mode is ever left.