Copy cell on middle click

[toby]

Feature request

Support copying the contents of a cell when the user middle-clicks on it.

Motivation

Middle-click is a common shortcut in terminal environments for paste (primary selection on X11). Mirroring that convention by having a middle click copy the cell under the cursor would make ad-hoc copy/paste workflows much faster — no need to enter a mode or press a keybinding first.

Proposed behavior

• Middle mouse button click on a cell copies that cell's contents to the clipboard (same path used by the existing copy keybinding).
• Provide brief visual/status feedback that the copy occurred (e.g. the same feedback shown for the keyboard copy action).
• No change to left/right click behavior.

Open questions

• Should the copy go to the system clipboard, the internal sheets register, or both?
• Behavior when middle-clicking outside the grid (headers, empty area) — likely a no-op.