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.

docs/claude/probe-readings.md

@@ -1229,3 +1229,17 @@ probe, exactly as the menu probe does it -- a popover dismisses itself when GTK
 grab goes missing, where the press has to close the bubble anyway. `click_gestures` reads that the controller
 is *connected* as well as that its handler works; the last press is on the button itself, which must be left
 to its own toggle (`want 1` rather than `0`).
+
+## `[drop-probe]` and what a refused drop says
+
+```
+[drop-probe] full row refused the drop: 0 (want 0)
+[drop-probe] refused feedback: notice='plot 1 holds four signals already: drop it on another plot, or add a row with the layout editor (L)' warning=1
+[drop-probe] the status line, composed: warning=1 status='plot 1 holds four signals already: ...   |   live · 9.97 kS/s · 2 s window'
+```
+
+`UDPSCOPE_PROBE_DROP_MS`. The first line is the refusal itself (a full row takes no more signals); the
+second is that it *said so* -- the message and the stock `.warning` class the status line carries while a
+notice is fresh; the third is the composed line the user reads, which is why the label is only checked here
+and not in the second: the status line folds the notice into itself on its own cadence, so a beat after the
+refusal it may still hold the previous text.