story: Draw the chart gallery through a virtual list

[huacnlee]

The Chart story built all 37 chart cards on every frame. Scrolling it cost about 20 ms a frame and dropped nearly every frame.

This flattens the gallery into rows of cards and hands them to gpui_kit::list, so only the rows near the viewport are built. Measured on the same window before and after:

	FPS	Frame	Dropped
Before	45	20.0 ms	100%
After	60	7.0 ms	0%

What changed

• Each chart is a ChartCard variant built on demand, and the fixture data moved behind one shared ChartData handle instead of being cloned once per chart on every frame.
• The groups are flattened into ChartRows of at most columns cards. The row width is measured during prepaint and the rows are re-chunked when it crosses a column boundary, so the cards keep the layout h_flex().flex_wrap() used to produce and still reflow when the panel is resized.
• The list owns the scroll, so the story sets paddings() to zero and carries the inset itself. That leaves the scrollbar on the panel edge, where the container used to put it.

No visual change: same cards, same order, same separators, same spacing.

Test plan

• cargo run -- chart, compared against main at the same window size: identical layout, and the FPS HUD numbers above.
• Scrolled to the end and confirmed the separators, candlestick row and both Sankey cards render.
• Resized the window narrower and confirmed the rows re-chunk.
• cargo clippy -p gpui-component-story --all-targets -- --deny warnings, cargo fmt --check, typos.

AI-generated with Claude Code.

🤖 Generated with Claude Code

https://claude.ai/code/session_01Fa8SBaJvfC5GsECZ6qjrFu