style(website): landing motion, interaction & layout polish

Feedback round (calmer, less "template/AI"):
- Dial back animated background motion globally (sway/leaf-drift halved,
  fewer falling leaves, gentler godrays/fireflies/light-sweep).
- Fire leaf confetti on click of real controls only (was pointerdown
  everywhere, which showered mobile users on every scroll).
- Remove hover lift/scale from non-interactive cards, screenshots and
  mascots so they no longer read as clickable.
- Fix mobile nav menu colours in the light theme (was a hard-coded dark
  panel with low-contrast links).
- Calm how-it-works steps 1-2 (no button pulse/shine/cursor bob); step 3
  keeps its lively sync animation. Remove the now-static "copied" toast
  from step 1 since it only made sense mid-animation.

UX audit follow-ups:
- Unify vertical rhythm with one section-padding scale; vertically centre
  the hero so space is balanced instead of pooling under the CTAs.
- Split the mixed "Why KoalaSync?" grid into use-case scenarios and a
  labelled "Features" subsection, and move the #features nav anchor there.
- Demote the hero GitHub CTA to a quiet text link (Chrome > Firefox >
  GitHub hierarchy); scale down oversized section mascots and give them a
  shared soft shadow.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

website/styles/demo.css

@@ -1147,12 +1147,8 @@ html.theme-light .hero-demo-scene .demo-video::after {
     font-size: 1.1rem;
     pointer-events: none;
     z-index: 10;
-    animation: pointer-click 1.6s infinite ease-in-out;
-}
-
-@keyframes pointer-click {
-    0%, 100% { transform: translate(0, 0) scale(1); }
-    50% { transform: translate(-3px, -3px) scale(0.9); }
+    /* Static hand — the looping tap bob was the busiest part of step 2 and
+       distracted from the copy (step 3 keeps its lively animation). */
 }
 
 /* Step 3 forest movie continuous animation bindings */