docs(cli): make Slider/Selector/MultiSelector showcases interactive

[durvesh1992]

Summary

Closes #3187. Closes #3188. Closes #3189.

The Slider, Selector, and MultiSelector showcase blocks rendered controlled components with a static value and a no-op/missing onChange, so the docsite previews appeared frozen — dragging the slider or picking an option did nothing. Wired local useState + onChange in each, mirroring the existing examples that already do this (SliderWithMarks, SelectorWithSections, MultiSelectorColumnVisibilitySelector).

• Slider/SliderShowcase.tsx — useState(50) + onChange
• Selector/SelectorShowcase.tsx — useState<string | undefined>() + value/onChange
• MultiSelector/MultiSelectorShowcase.tsx — useState<string[]>([]) + onChange; added 'use client' (now uses state)

Testing

• Regenerated docsite data — all three blocks parse and copy cleanly.
• Patterns are identical to existing stateful examples in the same folders (which compile), so the controlled-state wiring is type-correct.

I can't launch a browser locally to click-test the previews, but this is the exact state-wiring the issues request; a quick visual confirm on the preview is welcome.

Changeset

Included (@astryxdesign/cli patch) — showcase blocks ship in cli/templates.

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