fix: 🐛 stabilize widget lifecycle, renderer, and input validation

Overhaul createWidgetLoader to correctly handle widget reuse across story
re-renders:
- await widget.unmount() before switching stories
- treat null/missing args.widget as an early-return { widget: null }
- preserve state and props keys that are absent from story args instead of
  forcing empty defaults
- branch on available widget APIs (setProps / setState) to avoid clobbering
  load()-generated state when reusing the same story
- clear stale state keys before calling setState (both setters present) so
  Storybook-provided state replaces rather than merges with load() output
- guard against missing component plugin with a descriptive error

Rewrite createVanillaRenderer with a per-widget WeakMap so multiple
concurrent stories don't share render references:
- add strict validation for options and ViewComponent
- throw descriptive errors for ambiguous or non-callable component entries
- guard string-key lookup to object-only ViewComponent maps
- normalize null state/props to {} before calling widget setters
- update(widget) now accepts the widget instance to target the correct
  container across multiple rendered widgets

Add widgetProperties name/version validation to createPreviewConfig.

Additional fixes:
- fix(create-widget): correct className → class in vanilla ErrorView template
- docs(storybook): update Storybook install commands to v9, switch to CSF3
  object format with shared render, use Preact automatic JSX runtime,
  clarify two component-passing patterns for createVanillaRenderer, remove
  redundant widget argument from onClick/onReset event handlers

Tests: expand suite with 30+ cases covering all new branches in
createWidgetLoader, createPreviewConfig, and createVanillaRenderer.

Author: Jiří Fencl