Add shared UI action model and generated panel help

[raiseCatError]

Implements #166.

Stacked on #203 (feature/173-deterministic-presentation). Merge #203 first, then retarget this PR to dev.

What changed

• src/ui/actions.ts: a small UiAction model (stable id, label, help key label, bound key kinds, enabled state) with resolveAction, enabledActions and renderActionHelp. Footers list only enabled actions.
• Command palette: Enter dispatch and its footer come from paletteActions; Run and move are hidden when there is nothing to run or move over.
• Layout, syntax and transcript panels share DRAFT_PANEL_ACTIONS instead of three copies of the same footer.

Scope

• Behavior and key bindings are unchanged; no remappable bindings and no new shortcuts.
• Every action has a keyboard binding; nothing is pointer-only.
• Settings, prompt and provider panels keep their existing footers for now.

Verification

Focused tests in tests/uiActions.test.ts (disabled actions, keyboard completeness, narrow width, palette help). Full suite 625/625, typecheck, build and git diff --check clean.