Petals: saturated blossom tints + glossy shading

The pale pastel tints washed out to grey over the cream canvas. Use
saturated cherry-blossom colors, add a luminous centre + soft base
shadow per petal, and render at full opacity so they read as colorful.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

web/src/index.css

@@ -388,7 +388,7 @@ button, a, input {
    the writing. It hides itself in print and is skipped entirely for users who
    prefer reduced motion (the component renders nothing in that case). */
 .petal-fall {
-  opacity: 0.85;
+  opacity: 1;
 }
 
 /* Honor reduced-motion globally: still the looping ambient animations and drop