Tablet power features: keyboard shortcuts, drag-and-drop, multi-window #44

Part of #52 (Post-v1 upgrade track). Post-v1, not scheduled — do not start before the owning v1 workstream ships unless the epic reprioritizes it.

Outcome

Tablet power features on top of the v1 adaptive layout: keyboard shortcuts, drag-and-drop (in/out of the app), and multi-window/split-screen correctness.

Acceptance criteria

• DoD: verify green; tablet recordings of drag-and-drop and shortcut-driven navigation attached