Improve physics accuracy and enhance dark UI theme

[Claude]

The simulations used oversimplified physics (simple Euler integration, linear plastic models) and the UI had visibility issues (overlapping elements, insufficient contrast, heavy visual effects).

Physics Improvements

Integration Method

• Replaced forward Euler with Velocity Verlet in physics.js for 2nd-order accuracy and better energy conservation
• Added stepEuler() for backwards compatibility

Material Behavior Models (material-physics.js)

• Ludwik-Hollomon power law: σ = K·εⁿ for realistic strain hardening
• Ramberg-Osgood equation for elastic-plastic transitions
• Considère criterion for necking with actual cross-section area reduction
• Euler buckling analysis using critical load Pcr = π²EI/(KL)² with slenderness checks
• Basquin fatigue life with Goodman mean stress correction

Simulation Integration

• Tensile test: Newton-Raphson solver for strain from stress using Ludwik-Hollomon
• Compression test: Buckling triggers when F > Pcr instead of arbitrary thresholds
• Necking now updates crossSectionArea affecting stress calculations

Example from sim-tensile.js:

// Old: oversimplified plastic modulus
const plasticStrain = (stress - yield) / (E * 0.1);

// New: realistic strain hardening
const stressFromStrain = (strain) => MaterialPhysics.ludwikHollomonStress(strain, material);
// Newton-Raphson to find strain that produces current stress

UI Enhancements

Dark Theme

• Backgrounds: #08090b (was #0c0d10)
• Borders: opacity 0.05 (was 0.06)
• Accent: emerald #10b981 (was bright green #22c55e)
• Removed backdrop-filter: blur() for performance

Layout Fixes

• Notification container uses calc() for dynamic positioning below info-popup
• Mobile spacing increased to 20px (was 10px)
• Specimen-info constrained with max-width: calc(50% - 30px)
• CSS variables added for collision prevention

Visual Simplification

• Flattened buttons and panels (reduced shadows)
• Cleaner borders and transitions
• Consistent spacing via CSS variables