signal page: the header names it, numbered; no duplicate title or back button

The slide-over's header already prints the open page's title beside its Close button, so the
signal page's body carried the name twice and a "Back to signals" button that only repeated
navigation the header owns. Both are gone.

The header's title is now the **numbered** name -- `<n>. <name>`, `measure_channel_number()`'s
number, the same one the signal badges and the measurements pane use -- so the panel says
*which acquired channel* it is and not only what it is called. `slideover_set_title()`
retitles a page in place (the open one included): the number follows the stream's channel
numbering, and the page is refilled whenever the model's signal list changes.

Probe: `[signalpage]` reads `title='1. sine' title_ok=1` (the header text against the numbered
name) and `no_dup=1` (no "Back to signals" button and no `title-3` label in the body); the old
`back_to_list` reading is gone with the button. Help, README and the probe doc follow.

src/ui_slideover.cpp

@@ -118,6 +118,23 @@ bool slideover_add_page(App& app, const std::string& id, const std::string& titl
   return true;
 }
 
+bool slideover_set_title(App& app, const std::string& id, const std::string& title) {
+  SlideOver& s = app.slideover;
+  if (s.stack == nullptr || title.empty()) return false;
+  GtkWidget* child = gtk_stack_get_child_by_name(GTK_STACK(s.stack), id.c_str());
+  if (child == nullptr) return false;
+  GtkStackPage* page = gtk_stack_get_page(GTK_STACK(s.stack), child);
+  gtk_stack_page_set_title(page, title.c_str());
+  if (is_source_page(id)) {
+    AdwToggle* t = adw_toggle_group_get_toggle_by_name(ADW_TOGGLE_GROUP(s.source_switch), id.c_str());
+    if (t != nullptr) adw_toggle_set_label(t, title.c_str());
+  }
+  // The header shows the *open* page's title, so a retitle of the page on show is visible at once --
+  // a signal page's channel number can change under it when the stream renumbers its channels.
+  if (s.page == id) gtk_label_set_text(GTK_LABEL(s.title), title.c_str());
+  return true;
+}
+
 bool slideover_has_page(const App& app, const std::string& id) {
   return app.slideover.stack != nullptr &&
          gtk_stack_get_child_by_name(GTK_STACK(app.slideover.stack), id.c_str()) != nullptr;