fix: page transition UI jitter — remove enter phase to prevent double animation

The three-phase exit→enter→idle flow caused a visible "double refresh"
jitter. During the enter phase (220ms), the wrapper animated from
opacity:0 while cancelling child .page-motion with animation:none
!important. When phase switched to idle, the !important rule was
removed and child .page-motion re-triggered, creating a second
entrance animation — the jitter.

Fix: remove the enter phase entirely. After exit animation (180ms),
phase goes directly to idle. The child page's own .page-motion class
handles entrance naturally via React's fresh DOM mount. No wrapper
animation on enter, no double-animation conflict.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/components/PageTransition.tsx

@@ -6,7 +6,6 @@ interface PageTransitionProps {
 }
 
 const EXIT_DURATION_MS = 180;
-const ENTER_DURATION_MS = 220;
 
 const NAV_ORDER: string[] = [
   "home",
@@ -40,7 +39,7 @@ function getNavIndex(key: string): number {
 
 export default function PageTransition({ viewKey, children }: PageTransitionProps) {
   const [displayedChildren, setDisplayedChildren] = useState(children);
-  const [phase, setPhase] = useState<"idle" | "exit" | "enter">("idle");
+  const [phase, setPhase] = useState<"idle" | "exit">("idle");
   const [exitDirection, setExitDirection] = useState<"forward" | "backward" | "neutral">("neutral");
   const prevKeyRef = useRef(viewKey);
   const timerRef = useRef<ReturnType<typeof setTimeout>>();
@@ -73,38 +72,15 @@ export default function PageTransition({ viewKey, children }: PageTransitionProp
     setPhase("exit");
     timerRef.current = setTimeout(() => {
       setDisplayedChildren(children);
-      setPhase("enter");
+      setPhase("idle");
     }, EXIT_DURATION_MS);
     return () => clearTimeout(timerRef.current);
   }, [viewKey, children]);
 
-  // After enter animation completes, go back to idle
-  useEffect(() => {
-    if (phase !== "enter") return;
-    const timer = setTimeout(() => setPhase("idle"), ENTER_DURATION_MS);
-    return () => clearTimeout(timer);
-  }, [phase]);
-
   const dirClass = exitDirection === "forward" ? " is-forward" : exitDirection === "backward" ? " is-backward" : "";
 
-  if (phase === "exit") {
-    return (
-      <div className={`page-transition-wrap page-motion--exit${dirClass}`}>
-        {displayedChildren}
-      </div>
-    );
-  }
-
-  if (phase === "enter") {
-    return (
-      <div className="page-transition-wrap page-motion--enter">
-        {displayedChildren}
-      </div>
-    );
-  }
-
   return (
-    <div className="page-transition-wrap">
+    <div className={phase === "exit" ? `page-transition-wrap page-motion--exit${dirClass}` : "page-transition-wrap"}>
       {displayedChildren}
     </div>
   );