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.cpp

@@ -4009,10 +4009,16 @@ gboolean tick(gpointer user) {
   }
   if (glance.size() > app->glance_max_chars) app->glance_max_chars = glance.size();
 
+  // A notice is not the glance. It is prefixed onto the same line, which is exactly where a reader
+  // skimming the routine text will not see it -- "there is no clear feedback" was that, with a refusal
+  // ("plot 2 holds four signals already") looking like part of the status. The stock `.warning` class is
+  // what makes a message look like a message, and it goes when the notice does.
   if (!app->notice.empty() && g_get_monotonic_time() < app->notice_until_us) {
+    gtk_widget_add_css_class(app->status_label, "warning");
     const std::string with_notice = app->notice + "   |   " + glance;
     gtk_label_set_text(GTK_LABEL(app->status_label), with_notice.c_str());
   } else {
+    gtk_widget_remove_css_class(app->status_label, "warning");
     app->notice.clear();
     gtk_label_set_text(GTK_LABEL(app->status_label), glance.c_str());
   }