feat(layout): view panes from different workspaces or PCs side by side #904

Split from #792: main now shows two tabs side by side in one tab layout (#861), on top of herdr's own split layout (#726) and PaneCanvas drawing every pane of the active tab (#747).

Remaining

• Show panes from different workspaces together in the browser, not only tabs of the current workspace.
• Show panes from different PCs together. feat(split): two panes side by side, by dragging a tab or a sidebar row #602 proposed a second PaneTerminal under its own MachineContext for a half.
• Decide the scope beyond two halves: whether a grid with presets (feat(layout): arrange multiple pane views with docking presets #657) is wanted. feat(layout): arrange multiple pane views with docking presets #657 proposed a Running/All filter, six layout presets, stable number/colour badges, a lens per view, size and maximize (Mod+Shift+E), close-view-only, and a bulk chat/terminal switch.
• Define how attachments, leases and pending input behave when panes from several workspaces or PCs are visible. One WebSocket per PC is already kept (feat(ui): align pane controls and navigation with herdr TUI #747).
• Drag discoverability, as @flam-ing asked: drop zones that appear when a drag starts, a preview on hover with commit only on drop, Alt+drag if it clashes with sidebar reordering, and Esc to cancel (see fix(split): show early guides and smooth the drag preview Yoonwoo-Ha/herdr-web-ui#1).
• Phones: show the active view alone.

Please agree the direction here before opening a PR.