fix: the [edit] buttons' icons, and the locked pane says four lines not one

Two things the lock shipped wrong.

**The "done" icon did not exist.** `emblem-ok-symbolic` is a breeze icon; Adwaita — which is what this
app runs under — does not have it, so the button that leaves configuration mode drew a missing-image
placeholder, i.e. nothing. Both names are now Adwaita's own: `document-edit-symbolic` (the pencil: edit)
and `object-select-symbolic` (the check: done). This is exactly the silent failure `toolbar icons:`
exists for, so the two `[edit]` buttons get the same treatment: `pane edits: locked=…(ok) config=…(ok)`
is read by clicking into configuration mode and back, so one line carries both names and
`gtk_icon_theme_has_icon()`'s verdict for each. The invariants now carry the rule, including the trap
that a name found under `breeze` is not an answer.

**The locked pane was too compact.** One line could not hold what a user opens the pane for, so it is now
a small definition list: `signal`, `condition`, `level`, `mode` on four lines, and after that only the
figures that are *not* at their defaults — `noise band`, `HF reject`, `pre-trigger`, `hold-off` — because
a line reading "band 0" is a line that says nothing. The channel is resolved from the acquired channels
and the condition quotes the figures its own shape reads (a width and its comparison, two levels, or a
timeout), never one it ignores. The pane is ~192 px tall locked against 580 in configuration mode.

One self-inflicted critical on the way: the probe read the summary with `gtk_label_get_text()`, and the
container is a grid now — `Gtk-CRITICAL: assertion 'GTK_IS_LABEL (self)' failed`. The text comes from the
sync that writes it instead.

Verified: `pane edits: locked=document-edit-symbolic(ok) config=object-select-symbolic(ok)`;
`panes: trigger=locked signals=locked body_visible=0 summary='signal: square | condition: window, falling
| level: 0.3 .. 0.6 | mode: single | noise band: 0.2 | pre-trigger: 20 % | hold-off: 1000 ms'`;
`make test-ui` 14 files; zero `Gtk-CRITICAL` and no missing icon.

src/ui.cpp

@@ -4410,8 +4410,11 @@ void apply_pane_lock(App& app) {
   }
   const auto edit_button = [](GtkWidget* button, bool locked, const char* what) {
     if (button == nullptr) return;
+    // Both names must be ones Adwaita *has*: `emblem-ok-symbolic` exists only in breeze, so the "done"
+    // button was blank -- and a blank icon button is invisible in every other reading, which is why
+    // `pane edits:` checks these two by name and verdict.
     gtk_button_set_icon_name(GTK_BUTTON(button),
-                             locked ? "emblem-system-symbolic" : "emblem-ok-symbolic");
+                             locked ? "document-edit-symbolic" : "object-select-symbolic");
     const std::string tip = std::string(locked ? "configure " : "keep this configuration of ") + what;
     gtk_widget_set_tooltip_text(button, tip.c_str());
   };
@@ -5775,6 +5778,14 @@ int run_ui(int argc, char** argv, const Config& cfg, ScopeModel& model, Acquisit
                 app.dbg_trigger_sensitive, app.dbg_switcher_visible,
                 rows.cursors_visible() ? 1 : 0, rows.time_window(),
                 app.window_limit_s);
+    {
+      GtkIconTheme* theme = gtk_icon_theme_get_for_display(gdk_display_get_default());
+      std::printf("[udpscope]   pane edits: locked=%s(%s) config=%s(%s)\n",
+                  app.dbg_pane_edit_icon.c_str(),
+                  gtk_icon_theme_has_icon(theme, app.dbg_pane_edit_icon.c_str()) ? "ok" : "MISSING",
+                  app.dbg_pane_view_icon.c_str(),
+                  gtk_icon_theme_has_icon(theme, app.dbg_pane_view_icon.c_str()) ? "ok" : "MISSING");
+    }
     std::printf("[udpscope]   panes: trigger=%s signals=%s body_visible=%d summary='%s'\n",
                 app.dbg_trigger_locked ? "locked" : "config",
                 app.dbg_signals_locked ? "locked" : "config", app.dbg_trigger_body_visible,