Feature: Add "Take Screenshot" button in settings panel

[skkdevcraft]

Add a "Take Screenshot" control to the settings panel that exports the current WebGL canvas as a PNG (study-{date}.png), excluding UI overlays.

Requirements:

1. Register an 'action'-type setting (or equivalent) in the settings registry so the button is auto-generated alongside other controls.
2. Wire it to downloadScreenshot() from src/viewer/screenshot.ts.
3. Follow the settings architecture — no manual URL handling, no ad-hoc UI.
4. The button should be a native (44×44px minimum touch target, mobile-friendly).
5. Screenshot captures only the canvas — UI elements excluded (as specified in docs).

Already in place:

• src/viewer/screenshot.ts implements captureScreenshot() and downloadScreenshot()
• Settings registry (src/settings/registry.ts) and UI generation (src/settings/ui.ts) are ready for a new entry
• The type field may need 'action' added to SettingDefinition