P2: Add spring transitions to filter chip toggling

[hasitpbhatt]

Problem: Filter chips snap instantly between active/inactive states with no animation. Apple's buttons use smooth transitions with color interpolation.

Fix:

• Add CSS transition with cubic-bezier spring easing to .filter-chip
• active state should interpolate background, border-color, and text-color over ~250ms
• Use a spring curve like cubic-bezier(0.22, 1, 0.36, 1)

Files to modify:

• css/style.css — .filter-chip transition property