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.

docs/superpowers/plans/2026-10-11-ui-files-and-gtk4-practice.md

@@ -74,6 +74,10 @@ needs a probe that proves it *loads* and that its contents match whatever drives
 | `src/ui/ui_layout.cpp` (~20 kB) | the layout editor's overlay buttons, which are positioned by code | only the buttons are template |
 | `build_toolbar` (`src/ui.cpp`, ~212 lines) | the header bar's layout, with every button still wired in code | delicate: the `[udpscope] header:` probe reads its geometry |
 
+**Done since:** the sidebar is an `AdwOverlaySplitView` (`src/ui/main-window.ui`) rather than a paned's end
+child, so its width is its own 320 px and it overlays the plot instead of collapsing to zero — see
+`[sidebar-probe]`.
+
 Each of these is one change on the pattern `src/ui/ui_cursors.cpp` now follows: `ui_load()` the file, resolve
 the ids, keep every value, signal and rebuild in code, and run the panel's own probe before committing.