Add a contextual key-binding help footer

[AlexSkrypnyk]

Summary

Add a footer (and optional overlay) that lists exactly the keys valid for the currently active widget or panel.

Details

The TUI surfaces only fixed, partial key hints.

Add a context-aware help line, driven by the active widget, that shows the keys valid right now: a select shows filter/move/accept; a multiselect adds toggle and select-all; a text field shows caret/accept; the panel hub shows navigation and quit. Optionally expose the full list through a ? overlay. This makes the form self-teaching, especially for the less-common widgets.

Acceptance criteria

• A footer shows the key bindings valid in the current context, updating as focus moves between the panel hub and each widget editor.
• Each widget contributes its own relevant bindings.
• An optional ? key toggles a fuller help overlay.
• The help line is theme-driven and works in Unicode and ASCII modes.
• The footer can be turned off via a form-level option.
• Unit tests cover the per-context binding lists.
• The feature is documented in the README navigation section.