badges: hide the Source badge when the source has no panel of its own

A Source pill that opens nothing (UDPS, Flat UDP, files) was noise. It is now shown only while the slide-over has a source page; the [badges] source line gains visible=.

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

src/ui_app.hpp

@@ -160,7 +160,7 @@ struct BadgeBar {
   GtkWidget* signals_slot = nullptr;  ///< left; empty in step 1, step 2 fills it
   GtkWidget* system_slot = nullptr;   ///< right: Trigger, Source
   Badge trigger;
-  Badge source;                       ///< opens the first source page; a read-out when the source has none
+  Badge source;                       ///< opens the first source page; hidden when the source has none
   GtkWidget* signals_row = nullptr;   ///< the box inside the scroller: signal badges, then the "+"
   /// One per shown enabled signal; empty when overflowed. `unique_ptr`: each button's "clicked" handler holds
   /// a pointer to its `Badge`, which must not move when the vector does.