Use a light tooltip theme

[jbkunst]

What changed

• Changed the Lorenz input tooltips from the default dark style to a light theme.
• Tooltips now use a white background, dark body text, full opacity, a subtle border, and a small shadow.
• Updated the README so future tooltip implementations follow the same light visual treatment.

Scope

Only lorenz-attractor/app.R and README.md are modified. Tooltip content, input behavior, and interaction triggers are unchanged.

Validation

The styling uses the tooltip Sass variables supported by bslib::bs_theme() and a small .tooltip-inner rule for the border and shadow.