ui: the cursors and measure pages are insensitive until the stream has signals

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/ui_app.hpp

@@ -521,6 +521,8 @@ struct App {
   GtkWidget* cursors_reset = nullptr;
   /// The cursors panel's root, kept because cursors-off disables it rather than hiding it.
   GtkWidget* cursors_panel = nullptr;
+  /// The measurements page's root: like `cursors_panel`, insensitive until the stream has signals.
+  GtkWidget* measures_panel = nullptr;
   /// The panes' *own* switches, in their header rows, and the panel bodies those switches disable.
   /// The switch and the header bar's button are one control with two places to click: each runs the
   /// same action, and `sync_panel_switches()` re-reads both from the toolbar's state after any
@@ -1171,6 +1173,8 @@ struct App {
   /// not until the stream has signals), its width, and how many `GtkScrolledWindow`s it holds directly.
   int dbg_dock_shown = -1;
   int dbg_dock_sensitive = -1;
+  /// The Cursors and Measurements pages' sensitivity (1 only if both are), read on the tick.
+  int dbg_pages_sensitive = -1;
   int dbg_dock_w = -1;
   int dbg_dock_scrollers = -1;
   int dbg_trigger_sensitive = 0;