Keyboard navigation within lists (j/k, x to select, e to open)

[sjcotto]

We have global shortcuts (⌘K, /, ?, c) via components/shortcuts-help.tsx + the AppShell key handler. Add row-level navigation in the List view.

Scope

• j/k move the focused row, x toggles selection, e/Enter opens the task, space cycles status (optional).
• Respect the existing typing guard (don't hijack inputs).
• Document the new keys in the shortcuts dialog.

Acceptance

• Navigate and act on tasks without the mouse in the List view.