Detach a tab to a new window (drop outside termherd)

[bastien-gallay]

What

Using the same tab-drag gesture as #-move, dropping a tab outside any
existing termherd window spawns a new window hosting that tab.

Blocked by #149 (multi-window support); shares drag plumbing with the
cross-window move.

Current state

In-window tab drag exists (TabDrag crates/app/src/shell.rs:267,
Workspace::move_tab crates/core/src/workspace.rs:160). "Detach" = the drop
lands on no window → open a new one and re-home the tab there.

To do

• Detect a drop outside all windows; open a new window (needs Multi-window: open a new termherd window (mod+shift+n) #149).
• Re-home the session actor into the new window.
• Decide placement of the new window (cursor position?).

[bastien-gallay]

Refactor prerequisites: #168 (C2 view/tabs.rs — drag surface) and #171 (F3 window_geometry.rs — placement).