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/signals-panel.ui

@@ -37,7 +37,7 @@
         </child>
         <child>
           <object class="GtkButton" id="signals_edit">
-            <property name="icon-name">emblem-system-symbolic</property>
+            <property name="icon-name">document-edit-symbolic</property>
             <property name="valign">center</property>
             <property name="css-classes">flat</property>
           </object>