feat: the cursors panel is a UI file

src/ui/cursors-panel.ui holds the panel's shape -- the header with the mode dropdown and the reset
button, and the grid the two cursor times are read in. What is not in it is the grid's contents,
which follow the cursor mode and the rows and are rebuilt whenever either changes.

Panels need a builder that outlives them, unlike the dialogs, which are rebuilt on every opening: a
widget belongs to the builder that built it, so App::ui_builders holds each panel's for the life of
the window. ui_load() is that, in ui_widgets.cpp.

make test-ui also checks that udpscope-ui.gresource.xml lists every .ui file now. That was the third
trap of this refactor -- a file on disk but not in the resource is a Gtk-ERROR at load, which aborts
-- and it is a compile-time mistake that nothing else in the build could see. Verified by unlisting
the new file and watching the check fail, then restoring it.

[cursors-probe] reads as it did: all four modes, pane=1, the timings and the drag cases.

todo.md

@@ -30,5 +30,6 @@
   - the help window's chrome is **done** (`src/ui/help.ui`: `AdwWindow` + header bar + `AdwNavigationSplitView`)
   - the host form is **done** (`src/ui/source-form.ui`); the source chooser's own window is next
   - the main window's skeleton is **done** (`src/ui/main-window.ui`: the window, the two panes, the plot column and the status line, the sidebar's scroller); the panels inside it are the next slices
+  - the side panels are being extracted one per commit on that pattern: the cursors panel is **done** (`src/ui/cursors-panel.ui`), and the measurements panel, the trigger panel, the signals panel, the math panel, the events pane, the layout editor's buttons and the header bar are queued in the plan
   - next: the preferences (`AdwPreferencesDialog` + `AdwEntryRow`/`AdwSpinRow`/`AdwSwitchRow`), the help window's chrome, the source chooser and its host form, then the main window's skeleton
   - plan, conventions and acceptance for each slice: `docs/superpowers/plans/2026-10-11-ui-files-and-gtk4-practice.md`