fix(wbconf): add a header bar for a proper draggable titlebar

[ebenali]

Symptom

Dragging wbconf "by the titlebar" made the window expand/"mushroom" and lose its titlebar — while other apps (Chromium, pcmanfm) were fine.

Diagnosis

With temporary instrumentation I confirmed wbconf is pure client-side decorated: want=0, no waybox frame, deco=(nil), GTK shadow-offset geometry (+14+12). GTK4 does not implement the server-side xdg-decoration protocol, so waybox correctly leaves GTK windows CSD (the SSD gating added earlier). So the misbehaviour is GTK-side, not a waybox frame.

The real gap (as you suspected — "incompleteness in wbconf"): wbconf is a GtkApplicationWindow with no titlebar widget. A decorated GtkWindow with no header bar falls back to a degenerate CSD titlebar that misbehaves under a wlroots compositor and has no proper drag/maximize affordance.

Fix

Give wbconf an explicit GtkHeaderBar via gtk_window_set_titlebar(). GTK then owns a real, draggable titlebar (a GtkWindowHandle) that drives the compositor's interactive move/maximize through the normal xdg_toplevel requests — the standard GTK4 app structure.

On your "drive a seat from the cmdline" question

Yes — that's what the Wayland virtual-input protocols are for. I prototyped a zwlr_virtual_pointer_v1 press/move/release CLI to drive drags. The blocker for reproducing this headlessly turned out to be GTK's renderer losing its surface under the headless backend (VK_ERROR_SURFACE_LOST_KHR); GSK_RENDERER=cairo makes headless GTK render, but the CSD move gesture still doesn't replay reliably. I've filed a todo to generalise that prototype into a committed, reusable scriptable-input test helper.

Testing

• Full suite 27 green (g++/ASan); clang + release clean.
• wbconf maps cleanly under headless waybox with the header bar, no ASan/GTK errors.
• Interactive drag couldn't be reproduced headlessly (GTK renderer limitation noted above); this is the standard GTK4 pattern and the missing piece the symptom points to — please verify on real hardware.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com