Celebrate a logged pee/poo with emoji confetti

Tapping the pee or poo quick-log button now sets off a short burst of 💧/💩
particles from the button — pure DOM + CSS, pointer-events:none so it never
blocks taps, particles self-remove on animation end. It honours
prefers-reduced-motion, and there's a "Pee/poo confetti" switch in Settings
(device-local, on by default) to turn it off.

src/style.css

@@ -1348,3 +1348,32 @@ section.collapsed > :not(h2) { display: none; }
   color: var(--muted);
   text-align: center;
 }
+
+/* ---- pee/poo confetti ---- */
+#confetti-layer {
+  position: fixed;
+  inset: 0;
+  pointer-events: none;
+  overflow: hidden;
+  z-index: 9999;
+}
+.confetti-piece {
+  position: absolute;
+  line-height: 1;
+  will-change: transform, opacity;
+  animation-name: potty-burst;
+  animation-timing-function: cubic-bezier(0.2, 0.6, 0.35, 1);
+  animation-fill-mode: forwards;
+}
+@keyframes potty-burst {
+  0%   { opacity: 0; transform: translate(-50%, -50%) scale(0.4) rotate(0deg); }
+  12%  { opacity: 1; }
+  100% {
+    opacity: 0;
+    /* fly out to (dx, dy) then keep falling (gravity) */
+    transform: translate(calc(-50% + var(--dx)), calc(-50% + var(--dy) + 150px)) scale(1) rotate(var(--rot));
+  }
+}
+@media (prefers-reduced-motion: reduce) {
+  .confetti-piece { display: none; }
+}