Normalize gravity scale so 1.0 maps to previous 2.0

[Testtesttestestes]

Motivation

• The simulation felt like “normal” gravity at gravityScale = 2.0 while 1.0 was too weak, so user-facing 1.0 should match the previous effective gravity.

Description

• Set the internal _gravityScale initial value to 2.0, add _gravityScaleNormalization = 2.0, and update export const setGravityScale to multiply the input by the normalization constant in src/sph/sph.js so the UI gravityScale is normalized.

Testing

• Ran npm run lint which completed successfully.
• Built the production bundle with npm run build which succeeded.
• Started the dev server and captured a headless UI screenshot via Playwright to validate the control and rendering (succeeded).

---

Codex Task