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/styles/components/page-transition.css

@@ -53,24 +53,4 @@
     opacity: 0;
     transform: translateX(16px);
   }
-}
-
-/* Enter: explicit wrapper entrance animation overrides child page-motion */
-.page-motion--enter {
-  animation: page-enter-fade 220ms ease both;
-}
-
-.page-motion--enter .page-motion {
-  animation: none !important;
-}
-
-@keyframes page-enter-fade {
-  from {
-    opacity: 0;
-    transform: translateY(6px);
-  }
-  to {
-    opacity: 1;
-    transform: translateY(0);
-  }
 }