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_probe.cpp

@@ -1696,6 +1696,12 @@ gboolean prefs_probe(gpointer user) {
   // the `.ui` -- the one mistake neither the build nor `GtkBuilder` sees is a missing id.
   const char* shown =
       a.prefs.dir_row != nullptr ? gtk_editable_get_text(GTK_EDITABLE(a.prefs.dir_row)) : "-";
+  // 4/4: every row's title carries a mnemonic (`_Directory`, `Keep _events`, ...), so a row that does not
+  // ask for `use-underline` draws the underscore instead of underlining the letter.
+  std::fprintf(stderr, "[prefs-probe] mnemonics=%d/4\n",
+               (row_underlines(G_OBJECT(a.prefs.dir_row)) + row_underlines(G_OBJECT(a.prefs.keep_row)) +
+                row_underlines(G_OBJECT(a.prefs.relative_row)) +
+                row_underlines(G_OBJECT(a.prefs.theme_row))));
   std::fprintf(stderr, "[prefs-probe] opened=%d rows=%d dir=%s store=%s\n",
                a.prefs.dialog != nullptr ? 1 : 0, a.prefs.dir_row != nullptr ? 1 : 0,
                a.prefs.dir_row != nullptr ? shown : "-",