fix: row titles mean their mnemonics instead of drawing the underscore

Fifteen display strings in the two preference forms were written as GTK mnemonics -- `_Directory`,
`_Server`, `Multicast _group`, `_Theme` and the rest -- but the property that makes GTK read them was
never set, so GTK drew the underscore literally: `_Directory` in the row, not a D with a line under it.

Two switches are involved, FALSE by default in each case. `GtkLabel`/`GtkButton` take `use-underline`;
the rows take `AdwPreferencesRow:use-underline`, "whether an embedded underline in the title indicates a
mnemonic". The two buttons in the source form already set it (so their mnemonics worked and showed no
underscore); the thirteen rows did not. All thirteen now do.

The row classes that carry the property are not all related -- AdwEntryRow has its own, while AdwSpinRow,
AdwComboRow and AdwSwitchRow get it through AdwActionRow -- so the probe reads it by name on a GObject
(g_object_class_find_property, then g_object_get) rather than casting to one row type; a cast to a
`GtkWidget*` where the type is only forward-declared does not compile at all.

`_Cancel` and `_Connect` both claimed C in the same form, so Alt+C could only reach one of them: that
marker was unreachable, which is worse than absent because it is in the source to be trusted. `_Connect`
becomes `Conn_ect`, freeing C for Cancel.

[prefs-probe] reads the count (4/4) on the real widgets; check_ui validates the property against the GIR
for every object in every .ui file, inherited properties included.

src/ui_app.hpp

@@ -13,6 +13,7 @@
 
 #pragma once
 
+
 #include <gtk4/gtk_plot_area.hpp>
 
 #include "acquisition.hpp"
@@ -33,6 +34,8 @@
 #include "status_text.hpp"
 #include "waveform.hpp"
 
+
+
 #include <adwaita.h>
 #include <gtk/gtk.h>