fix: the overlaid sidebar steps aside for a drag, and the compact view follows the trigger

Two reports, both real, and neither was about the lock itself.

**A drag out of an overlaid sidebar had nowhere to land.** In a window too narrow to show both columns,
`AdwOverlaySplitView` puts the sidebar *on* the plot — that is what `collapsed` means — so the pointer
stays over the list and never over a drop target: a signal could be dragged but not dropped, which reads
as "dragging does not work". `row_drag_sidebar()` now hides the sidebar on `drag-begin` and restores it on
`drag-end`, and only when collapsed: side by side the plot is already visible, and hiding the list the drag
started in would be startling. The reading begins a drag through the *source* with the state forced
collapsed, the way the sidebar probe forces it:

    pane drag: sidebar hidden for the drag=1 restored after it=1

**The compact view did not follow the trigger switch.** The locked line's first branch reads
`tc.enabled` — with trigger mode off it says `stream (trigger off)` and nothing else — and
`on_trigger_toggled()` never re-synced it, so the line kept saying whichever it had been. One call to
`sync_trigger_summary()` beside the switch sync. The dump's two `panes:` lines were already the test and
now pass it: with a session whose trigger is off, the line as found reads `stream (trigger off)` and the
live one reads the settings, because the exit probe turns the trigger on in between. Both reading `stream`
was the defect.

Verified: the two lines above; `panes: the same line as the probe found it: 'mode: stream (trigger off)'`
against the live `'signal: square | condition: window, rising | level: 0.3 .. 0.6 | mode: normal'`;
`make test-ui` 14 files; zero `Gtk-CRITICAL`. Docs: the `pane drag:` reading and the invariants bullet for
the overlay, and the `panes:` pair's second job — the refresh test — written down so the next reader knows
what both lines reading `stream` means.

src/ui_signals.cpp

@@ -457,6 +457,17 @@ void rebuild_signal_list(App& app, const std::vector<SignalInfo>& signals) {
     g_signal_connect_data(source, "prepare", G_CALLBACK(on_drag_prepare), ctx,
                           [](gpointer data, GClosure*) { delete static_cast<DragContext*>(data); },
                           G_CONNECT_DEFAULT);
+    // A drag out of an *overlaid* sidebar would have nowhere to land (see `row_drag_sidebar()`), so the
+    // sidebar steps out of the way for as long as the drag lasts.
+    g_signal_connect(source, "drag-begin", G_CALLBACK(+[](GtkDragSource*, GdkDrag*, gpointer user) {
+                       row_drag_sidebar(*static_cast<App*>(user), true);
+                     }),
+                     &app);
+    g_signal_connect(source, "drag-end",
+                     G_CALLBACK(+[](GtkDragSource*, GdkDrag*, gboolean, gpointer user) {
+                       row_drag_sidebar(*static_cast<App*>(user), false);
+                     }),
+                     &app);
     gtk_widget_add_controller(row, GTK_EVENT_CONTROLLER(source));
 
     gtk_list_box_append(GTK_LIST_BOX(list), row);