fix(registry): clear flowchart selection after click-away

[AksharP5]

What

The Flowchart previews keep their blue selection outline after the cursor clicks away. This clears the outline at the intended deselection beat in both horizontal and vertical blocks.

Fixes #3261.

Why

The blocks create the outline during playback, but GSAP resolves the later fade target while constructing the timeline. At that point the element does not exist, so the fade tween has no target.

How

Added the hidden outline to each block's initial markup and moved its reveal onto the existing GSAP timeline. The later fade now binds to the element and works during normal playback and timeline seeking. Regenerated the Catalog pages and payloads.

Test plan

• Unit tests added/updated
• Manual testing performed
• Documentation updated

Validated with:

• bun run build
• bun run lint:registry-items flowchart flowchart-vertical
• bunx oxfmt --check on all changed files
• hyperframes check against both standalone blocks at the selection and deselection states
• Browser playback of both generated Catalog payloads, confirming opacity changes from 1 while selected to 0 after click-away

Before and after

Vertical

flowchart-vertical-comparison.mp4

Horizontal

flowchart-comparison.mp4