feat: the main window's skeleton is a UI file

src/ui/main-window.ui holds the window, its two panes, the plot column with the status line under it,
and the sidebar's scroller. What stays in code is what depends on the stream: the events pane, the two
plot areas, the sidebar's panels and the status popover, each put into a slot by id.

The header bar is deliberately not in the file. It is the window's own titlebar, every button on it is
wired to an action, and an AdwApplicationWindow refuses a titlebar outright -- a fatal Adwaita-ERROR
rather than a warning that leaves the window merely ugly -- so the window is a GtkApplicationWindow and
the header bar stays where it was.

gtk_paned_set_position() is the other thing that had to stay in code, and the reason is order rather
than shape: it clamps against its children *now*, so stating it before the sidebar's panels exist moved
the plot column by ~180 px.

The layout, events and box-zoom probes read what they read before this change, and the header reads
window_button_w=130 bar_w=756 as it always has -- checked by building the previous commit and running
the same probe against it, because the layout numbers in the notes turned out to be stale from an
earlier session. Five UI files now.

src/ui/udpscope-ui.gresource.xml

@@ -13,5 +13,6 @@
     <file preprocess="xml-stripblanks">preferences.ui</file>
     <file preprocess="xml-stripblanks">help.ui</file>
     <file preprocess="xml-stripblanks">source-form.ui</file>
+    <file preprocess="xml-stripblanks">main-window.ui</file>
   </gresource>
 </gresources>