Add gravity strength slider for SPH water simulation

[Testtesttestestes]

Motivation

• Provide a user-controllable gravity multiplier so the water/SPH simulation can be tuned interactively.

Description

• Add a new Gravity UI slider in src/App.tsx (range 0..2, step 0.01) and pass the selected value as a gravityScale prop to RealisticProgressBar.
• Extend RealisticProgressBar (src/components/RealisticProgressBar.tsx) with a gravityScale prop, sync it via useEffect, and call SPH.setGravityScale(...) (also applied during renderer initialization).
• Add gravity-scaling support in the SPH engine (src/sph/sph.js): introduce _gravityScale state, export setGravityScale(scale), add helper _scaledGravity(), and use the scaled gravity for initial half-step velocity, velocity-limit / timestep calculation, and the per-frame g uniform sent to the particle update shader.

Testing

• Ran npm run build and the production bundle was built successfully.
• Launched the dev server with npm run dev and confirmed the app is reachable for visual checks.
• Executed an automated Playwright script that loaded the page and captured a screenshot (saved as an artifact) to visually verify the new Gravity slider and its UI integration.

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Codex Task