Tile across all screens at once.

**Tiling across multiple screens**
I have 3 32 inch monitors, so far i can split any of these monitors into multiple parts with this extension, but not join them together, for example an IDE might span over multiple monitors because I often have many files open in many tabs side by side... so making a window span over 2 monitors make sense here... I would like to have a way to define areas over all screens at once instead of screen by screen like now.

**Additional context**

No files, tests, or entry points are named. Start by tracing the existing screen-by-screen tiling flow and how layouts are defined for multiple monitors; done means users can define and apply one tiling area spanning selected screens while preserving current single-screen behavior.

Written by the indexing model from the issue text.