ui: a refused drop says so, and looks like a message rather than the status

The refusal was never silent -- `on_drop_channel()` has always set a notice -- but the
status bar renders `notice + "   |   " + glance` in one label, so "plot 1 already holds
four signals" arrived looking like part of the routine text a reader skims, which is the
"no clear user feedback" report. The probe's own reading showed it plainly: the label held
`plotting 'sine' on plot 1   |   live · 4.44 kS/s · 2 s window`.

Three changes:

- While a notice is fresh the status label carries the stock `.warning` class, so a message
  looks like a message and the class goes when the notice does. Every notice benefits, not
  just this one.
- The refusal says what to *do*: "plot 1 holds four signals already: drop it on another
  plot, or add a row with the layout editor (L)".
- The drop's inline notice now goes through `set_notice()`, the one setter, instead of
  writing `app.notice` and its deadline by hand.

[drop-probe] reads the whole path, with the row filled first so the drop under test is the
refusal:

    full row refused the drop: 0 (want 0)
    refused feedback: notice='plot 1 holds four signals already: ...' warning=1
    the status line, composed: warning=1 status='plot 1 holds four signals already: ... | live · 9.97 kS/s · 2 s window'

The label is read in the third line and not the second on purpose: the status line composes
the notice into itself on its own cadence, so a beat after the refusal it may still hold the
previous text -- which the first attempt at this reading mistook for the fix not working.

The row limit is not silent in the same way: the layout editor's `+` is insensitive at
`kMaxRows`, which is a control that says "no" by being unusable rather than a message.

src/ui_signals.cpp

@@ -290,8 +290,7 @@ gboolean on_drop_channel(GtkDropTarget*, const GValue* value, double x, double y
   if (signal >= app->model->signals().size()) return FALSE;
 
   const auto notice = [&](const std::string& what) {
-    app->notice = what;
-    app->notice_until_us = g_get_monotonic_time() + 4 * G_TIME_SPAN_SECOND;
+    set_notice(*app, what, 4);
     std::fprintf(stderr, "[udpscope] drop: %s\n", what.c_str());
     std::fflush(stderr);
   };
@@ -334,7 +333,11 @@ gboolean on_drop_channel(GtkDropTarget*, const GValue* value, double x, double y
     }
   }
   if (slot < 0) {
-    notice("plot " + std::to_string(row + 1) + " already holds four signals");
+    // Say what to do about it. A refusal a reader cannot act on is only half an answer: the two ways
+    // out are another plot or another row.
+    notice("plot " + std::to_string(row + 1) +
+           " holds four signals already: drop it on another plot, or add a row with the layout "
+           "editor (L)");
     return FALSE;
   }
   // The slot remembers the *signal* (and element), never the channel index: the