Split-screen: view two sessions side by side

[snazzybean]

Drag a session out of the sidebar into the terminal area and drop it next to another session (or, later, the file browser), with a preview box shown while dragging.

The drag-and-drop mechanic itself is a small, self-contained frontend piece. The real cost is in the terminal. Today there's exactly one #terminalFrame iframe, and everything js/terminal.js does around it (synthetic keyboard events, resize handling, focus routing, the tmux-buffer copy path used instead of OSC-52) assumes there's only one. Showing two sessions at once means running several ttyd iframes at the same time, each carrying that machinery on its own, plus deciding which pane owns keyboard focus and resize at any given moment.

A cheaper option that covers most of the actual want: a fixed two-column toggle (terminal and files side by side, no drag, no second terminal) instead of full multi-session split-screen.