fix: 🐛 make Showcase-wrapped widgets clickable

[RuslanTsitser]

Description

There was an issue where widgets wrapped with Showcase / Showcase.withWidget could become non-interactive (especially on desktop/web).

In these cases, the overlay layer (MouseRegion / Material) could end up capturing pointer events and preventing the underlying widget from receiving them. This meant the wrapped widget would not reliably get events like onTapDown and could fail to participate in the gesture arena (taps, swipes, drags).

This PR fixes that by:

• Setting opaque: false on MouseRegion in TargetWidget and TooltipWrapper, allowing events to pass through transparent areas.
• Using Material(type: MaterialType.transparency, ...) in FloatingActionWidget, which also prevents the wrapper layer from blocking pointer interactions (same practical effect as above, even though the exact underlying reason is not fully clear).

As a result, widgets inside Showcase behave as expected and remain clickable/gesture-responsive.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

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