feat: section body animation with blur-out, delayed enter, and animation tokens

Add animation tokens to :root (--ease-spring, --ease-decelerate,
--ease-default, --duration-fast/normal/slow/spring). Apply spring easing
to section title enter. Add separate section-body transition: fast
blur-out exit (100ms), clean slide-in enter (350ms) delayed by 200ms so
content appears after the title animation completes.

src/shared/styles/theme.css

@@ -75,6 +75,15 @@
 
   /* === GRID === */
   --grid-gap: var(--space-3);
+
+  /* === ANIMATION === */
+  --ease-default: ease;
+  --ease-spring: cubic-bezier(0.34, 1.56, 0.64, 1);
+  --ease-decelerate: cubic-bezier(0.25, 0, 0, 1);
+  --duration-fast: 100ms;
+  --duration-normal: 200ms;
+  --duration-slow: 350ms;
+  --duration-spring: 380ms;
 }
 
 @theme inline {
@@ -235,8 +244,8 @@
   opacity: 1;
   transform: translateY(0);
   transition:
-    opacity 0.35s ease,
+    opacity var(--duration-slow) var(--ease-default),
-    transform 0.35s ease;
+    transform var(--duration-slow) var(--ease-default);
 }
 
 @starting-style {
@@ -248,11 +257,17 @@
 
 /* Cross-section view transition (navigation between sections) */
 ::view-transition-old(section-content) {
-  animation: 200ms ease both section-fade-out;
+  animation-name: section-fade-out;
+  animation-duration: var(--duration-normal);
+  animation-timing-function: var(--ease-default);
+  animation-fill-mode: both;
 }
 
 ::view-transition-new(section-content) {
-  animation: 300ms ease both section-fade-in;
+  animation-name: section-fade-in;
+  animation-duration: var(--duration-spring);
+  animation-timing-function: var(--ease-spring);
+  animation-fill-mode: both;
 }
 
 @keyframes section-fade-out {
@@ -276,3 +291,41 @@
     transform: translateY(0);
   }
 }
+
+/* Section body: instant blur-out, clean slide-in */
+::view-transition-old(section-body) {
+  animation-name: section-body-out;
+  animation-duration: var(--duration-fast);
+  animation-timing-function: var(--ease-default);
+  animation-fill-mode: both;
+}
+
+::view-transition-new(section-body) {
+  animation-name: section-body-in;
+  animation-duration: var(--duration-slow);
+  animation-delay: var(--duration-normal);
+  animation-timing-function: var(--ease-decelerate);
+  animation-fill-mode: both;
+}
+
+@keyframes section-body-out {
+  from {
+    opacity: 1;
+    filter: blur(0);
+  }
+  to {
+    opacity: 0;
+    filter: blur(3px);
+  }
+}
+
+@keyframes section-body-in {
+  from {
+    opacity: 0;
+    transform: translateY(16px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}