fix(popover): stop animating left/top so first open doesn't slide from corner

src/shared/ui/Popover/Popover.svelte

@@ -88,6 +88,14 @@ let resolvedSide = $state(side);
  */
 let positioned = $state(false);
 
+/**
+ * Resolved fixed-position coordinates. Applied through the reactive `style`
+ * attribute (not imperatively) so they can't be wiped when the attribute
+ * re-renders — mixing the two caused a one-frame top-left flash.
+ */
+let x = $state(0);
+let y = $state(0);
+
 /**
  * Actual DOM open state, driven by the `toggle` event. Source of truth for
  * whether the browser currently shows the popover; `open` is the public binding.
@@ -132,8 +140,8 @@ function updatePosition(): void {
         sideOffset,
     });
     resolvedSide = result.side;
-    contentEl.style.left = `${result.x}px`;
-    contentEl.style.top = `${result.y}px`;
+    x = result.x;
+    y = result.y;
     positioned = true;
 }
 
@@ -204,9 +212,9 @@ $effect(() => {
     data-side={resolvedSide}
     data-state={shown ? 'open' : 'closed'}
     ontoggle={onToggle}
-    style={`position: fixed; inset: auto; margin: 0;${positioned ? '' : ' visibility: hidden;'}`}
+    style={`position: fixed; inset: auto; left: ${x}px; top: ${y}px; margin: 0;${positioned ? '' : ' visibility: hidden;'}`}
     class={cn(
-        'opacity-0 scale-95 transition-discrete transition-all duration-fast',
+        'opacity-0 scale-95 transition-discrete transition-[opacity,transform] duration-fast',
         'starting:opacity-0 starting:scale-95',
         '[&:popover-open]:opacity-100 [&:popover-open]:scale-100',
         'data-[side=top]:origin-bottom data-[side=bottom]:origin-top',