feat: refine demo themes and keep chart active beside settings

[rosek86]

Move the demo theme selector to the top-right header and refine the dark appearance with neutral surfaces, restrained colors, and simpler controls.

On desktop, Chart settings opens beside the chart with matching height. Zoom and markers remain active while the rest of the page is strongly dimmed and inert. Settings scroll internally when needed. Smaller screens retain a modal dialog, and changing viewport size switches modes without losing settings.

Clicking or tapping the dimmed background, Escape, and Close dismiss settings and restore focus to the opener. Clicking the chart or dragging out of the settings panel keeps settings open.

Align neighboring desktop panels in both rows. Apply the same graphite palette, typography, controls, borders, and spacing to the example gallery and every example, including the independently runnable basic consumer. Preserve explicit white hosts for light library charts.

Validation:

• npm run lint:fix, npm run format, and npm run check passed (465 unit tests).
• Full Chromium browser suite passed (125 tests).
• Settings interaction tests passed in Firefox and WebKit (5 each).
• Reviewed desktop and mobile screenshots, including the gallery and basic example; verified matching panel heights in both desktop rows.