fix: the sidebar keeps its own width, so a small window can no longer hide it

The sidebar's width was a GtkPaned divider position: `window - 940` in pixels. On any window
narrower than 940 px the divider landed past the right edge and the sidebar came up 0 px wide,
reachable only by dragging the divider back.

Every attempt that kept the paned failed for that reason -- deriving the position from the paned's
own width, re-parenting the column, property-form children, code-set size requests. The mechanism was
the fault, so it is gone: the row is an AdwOverlaySplitView, which is what a utility sidebar is in
GNOME, with min-sidebar-width = max-sidebar-width = 320 in pixels. Its width is therefore its own at
any window size, and when there is no room beside the plot it *overlays* the plot (collapsed, set from
the window's width in tick()) instead of being squeezed to nothing.

Two things found on the way, both recorded in the invariants:

- sidebar-width-fraction is a 0..1 fraction of the window whatever sidebar-width-unit says. Setting a
  pixel count there is a GObject critical and it falls back to 0.25, which is why the sidebar first
  read 560. The width is pinned with min/max instead.
- the window opened at its content minimum (756 px, the plot column plus the sidebar) because the
  explicit gtk_window_set_default_size() was dropped when the window moved into the .ui. Restored.

[sidebar-probe] reads both states and is the guard: window=756 sidebar=320 under a 2x scale, which is
the case that used to read 0.

src/ui_app.hpp

@@ -405,6 +405,10 @@ struct App {
 
   /// The main window's builder. Held because the window belongs to its object list: it is replaced on
   /// the next build rather than reused, and everything found by id comes out of it.
+  /// The split view the sidebar lives in. Its width, its collapse and its overlay are that widget's
+  /// business -- `AdwOverlaySplitView` -- and not a paned position's.
+  GtkWidget* side_view = nullptr;
+
   GtkBuilder* window_ui = nullptr;
   bool comparing = false;
   /// The last x/y range seen on each side of a comparison, one entry per row: a frame compares them
@@ -1000,6 +1004,9 @@ struct RowMenuProbeState {
 };
 
 constexpr int kTickMs = 16;
+/// Below this window width the sidebar has no room beside the plot, and the split view overlays it
+/// instead of squeezing it. See `tick()`.
+constexpr int kSidebarRoom = 900;
 constexpr int kWindowUnitCount = 3;
 constexpr double kWindowMinS = 1.0e-6;
 constexpr int kWindowLabelChars = 9;
@@ -1129,6 +1136,7 @@ void sync_events_divider(App& app);
 /// because the theme is the one preference that takes effect without a restart.
 void open_preferences(App& app);
 gboolean prefs_probe(gpointer user);
+gboolean sidebar_probe(gpointer user);
 gboolean boxzoom_probe(gpointer user);
 /// A labelled row whose control keeps its natural width -- what a `GtkSwitch` needs and what
 /// `labeled_box()`'s expanding child denies it.