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_app.hpp

@@ -269,7 +269,12 @@ struct App {
   /// session: a lock is a mode, not a setting, so it is not persisted.
   bool trigger_locked = false;
   bool signals_locked = false;
+  /// The locked trigger pane's content: a small definition list, because one line cannot hold the
+  /// signal, the condition, the level and the mode, and those are what a user reads the pane for.
   GtkWidget* trigger_summary = nullptr;
+  static constexpr int kTriggerSummaryRows = 8;
+  GtkWidget* trigger_summary_keys[kTriggerSummaryRows] = {};
+  GtkWidget* trigger_summary_values[kTriggerSummaryRows] = {};
   GtkWidget* trigger_edit = nullptr;
   GtkWidget* signals_edit = nullptr;
   int dbg_trigger_locked = -1;
@@ -278,6 +283,10 @@ struct App {
   /// The compact line the locked trigger pane shows. It is the pane's whole content in that mode, so it
   /// is read rather than assumed: empty here is a locked pane showing nothing at all.
   std::string dbg_trigger_summary;
+  /// The two [edit] buttons' icons, read in each mode by clicking into it and back: a button whose icon
+  /// does not resolve is *blank*, and that is how the missing "done" icon was found.
+  std::string dbg_pane_edit_icon;
+  std::string dbg_pane_view_icon;
   /// The cursors pane: listing what the instruments read -- T0/T1/dT always, per row an Y1/Y2/dY
   /// where rulers are up, and in `mix` mode the value each cursor crosses each trace at.
   GtkWidget* cursors_mode_drop = nullptr;