Split the sidebar into Chain and Setup tabs, with a draggable chain list

The sidebar grew crowded as the chain got longer. The chain and the params
of the selected stage now share a Chain tab through a vertical split the
user drags, and the sources, canvas and seed move to a Setup tab. Stage
rows are compact, with their move and remove buttons shown only on the
hovered, selected or focused row.

Add effect had stopped opening: its menu of fifteen effects no longer fit
below the button, and GTK does not show a popover that does not fit. The
menu now scrolls past 480 pixels, and the button sits beside the chain
heading, where the split cannot push it down the screen.

The window code becomes an effector_gui library so a new GUI test binary
can drive the real window under Xvfb; ctest skips it when xvfb-run is
missing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -12,7 +12,7 @@ ctest --test-dir build --output-on-failure
 
 Everything is warning-clean under `-Wall -Wextra -Wpedantic`; keep it that way.
 
-The test suite is a single hand-rolled binary (no framework), so there is no
+The core test suite is a single hand-rolled binary (no framework), so there is no
 per-test filter: run `./build/tests/effector_tests` directly to see the section
 headings (`rng`, `registry`, `scanlines`, `tiles`, `blur`, `chain`, `project`,
 `image io`, `batch loading`) and the `ok`/`FAIL` lines. To iterate on one area, comment
@@ -20,6 +20,13 @@ out the other blocks in `tests/core_tests.cpp` or add a new
 `std::cout << "section\n"` block — checks are plain
 `check(condition, "description")` calls and `main()` returns the failure count.
 
+The interface has its own binary, `./build/tests/effector_gui_tests`
+(`tests/gui_tests.cpp`, same `check()` style), which opens the real window and
+drives it by widget names and tooltips (`chain-list`, `add-effect`,
+`sources`, "Move up", ...). It needs a display: ctest runs it under `xvfb-run`
+when that is installed and otherwise reports it skipped (exit 77). To run it by
+hand, use the Xvfb invocation below.
+
 Headless render (no display needed, the fastest end-to-end smoke test):
 
 ```bash
@@ -148,8 +155,9 @@ is discarded rather than mixed into the new project (`dropLoadedImages_`).
 
 `effector_core` (static) holds image IO, params, effects, rng and links only
 `gdk-pixbuf-2.0`, `pangocairo` (the text effect's glyphs; Pango and Cairo need
-no display) + Threads — **no gtkmm, no display**. The `effector` executable
-adds `src/main.cpp`, `src/cli/`, `src/gui/` and links gtkmm. Keep GTK out of
+no display) + Threads — **no gtkmm, no display**. `effector_gui` (static) adds
+`src/cli/` and `src/gui/` and links gtkmm, and the `effector` executable is
+`src/main.cpp` on top of it; the GUI tests link `effector_gui` too. Keep GTK out of
 `src/core/` and `src/effects/`; the tests link the core alone and rely on this.
 
 ### Determinism is a contract