Make Separate All Panes and Separate Current Pane keybindable commands

[thdxg]

Stacked on #230 (which stacks on #228) — retarget as the PRs beneath merge.

Separate Panes existed only in the tab context menu. This PR makes it and a new per-pane sibling first-class commands — palette rows plus rebindable hotkeys (unbound by default, matching the other tree-rewriting actions like Apply/Save Layout):

• Separate All Panes — the context-menu action, invocable on the active tab.
• Separate Current Pane — new: splits just the focused pane out of the active tab into its own tab, inserted right after it.

Why commands and not a sidebar drag

Dragging a pane's grab handle onto the sidebar was tried first (twice) and is not implementable with public SwiftUI: the sidebar column's hierarchy never receives the grab handle's AppKit NSDraggingSession. Tested against the live app with synthesized drags — row-, ForEach-, and header-level dropDestination; row-, header-, and List-level onDrop; and a target outside the List — none ever got a single delegate callback, while the identical delegate fires fine in the workspace column (the same drag session reshapes splits there). The finding is recorded on MovablePane's doc comment so nobody retries it blind.

How it works

• AppState.separatePane does removePane's tree/zoom/focus repair minus the surface teardown, so the Pane object (surface and running shell) survives the trip; crossing projects rebinds routing identity the way moveTab does. The destination workspace resolves before detaching, so a failure can't leave the pane belonging to no tab.
• Both commands follow the add-an-action recipe: AppCommand cases (palette picks them up via allCases), HotkeyAction cases, and routing through the AppCommand.action(in:) single source of truth in Responders.swift, so palette, menu, and binding can't drift. Enablement guards make them no-ops on single-pane tabs.
• MovablePane stays as the shared drag-pasteboard decode for the workspace's LeafDropDelegate, which previously unpacked the UUID bytes inline.

Testing

• New AppStateTests: pane lands in its own tab with source-tab zoom/focus repair, insertion at a slot, cross-project rebind + destination activation, only-pane / unknown-pane no-ops. Full suite passes; format/lint clean.
• Live-verified through the command palette in a hermetic app instance: "Separate Current Pane" on a 2-pane tab produced the new tab right after the source with focus following; "Separate All Panes" exploded a 3-pane tab into single-pane tabs in place.

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