fix: drag the window from the sidebar's empty area

[lsoengas]

What this does

Right now the only way to move a Programa window is to grab the tiny strip next to the traffic-light buttons, which is uncomfortable. With this change you can click and drag the empty space in the sidebar (below the tab list) to move the window. Double-clicking that same area still creates a new workspace, exactly like before.

Summary

• SidebarEmptyArea now mounts the existing WindowDragHandleView as a frontmost .overlay. A .background placement does not work: the Color.clear + .contentShape content in front is the hit-test target, so the drag view never receives the click.
• WindowDragHandleView gains two opt-in parameters, handlesDoubleClick and onDoubleClick. With the drag view frontmost it claims the first click of any double-click, so a sibling .onTapGesture(count: 2) can never reliably count to two. The drag view now owns the whole double-click sequence and fires the add-workspace action directly. Both existing call sites (custom titlebar, sidebar top strip) use the defaults and keep byte-identical behavior.
• Drop-to-reorder into the empty area is unaffected: the drag view's hitTest bails for anything that is not a left mouse down, so drag sessions fall through to the existing onDrop target.
• Debug-build dlog instrumentation added at the capture/double-click decision points.

Note: dragging from the top tab bar's empty strip needs changes in the vendor/bonsplit submodule, which currently points at manaflow-ai/bonsplit (no push access). That part is intentionally left out of this PR until we decide whether to fork bonsplit into the org.

Test Plan

• Manual: drag from sidebar empty space moves the window
• Manual: double-click sidebar empty space still creates and selects a new workspace
• Manual: dragging a tab into the sidebar empty area still reorders
• Manual: titlebar and sidebar-top drag/zoom behavior unchanged
• CI

No automated regression test: this is AppKit hit-testing behavior that our unit environment cannot exercise end-to-end, per the repo test quality policy.