feat(web): use CSS linear() for spring animations

[janicduplessis]

Summary

• Replace hardcoded cubic-bezier(0.25, 0.46, 0.45, 0.94) spring approximation with real spring physics simulation
• Generate CSS linear() easing points from a damped harmonic oscillator (same physics as iOS/Android)
• Spring duration now derived from simulation settling time (incorporates stiffness, which was previously ignored)
• Lazy linear() feature detection with automatic cubic-bezier fallback for unsupported browsers
• Cache generated linear() strings by spring params to avoid recomputation

How it works

1. Simulate spring at 120Hz: a = (-stiffness * (x - 1) - damping * v) / mass
2. Downsample to 60fps, collect position samples
3. Format as linear(0, 0.15, 0.82, 1.12, 1.05, 0.98, 1.0)
4. Browser interpolates linearly between points → smooth spring curve

Test plan

• yarn example web — open per-property demo with spring transition
• Inspect element in DevTools — verify linear(...) in CSS transition
• Spring should visually bounce/oscillate (not just ease-in-out)
• Compare with native iOS/Android behavior