perf(calendar): stop mounting a HoverCard per day cell that can never open

Third instance of the same pattern, and the worst ratio yet. Mantine mounts a HoverCard's
Popover machinery - Popover, PopoverTarget and PopoverDropdown - even when it is disabled.
A month grid is around 42 cells and `disabled` is `isEditMode || eventsForDate.length === 0`,
so on a typical month most days mounted three components for a card that can never open, and
in edit mode every single day did.

Measured on a real board: 42 Popovers, 42 PopoverDropdowns and 42 PopoverTargets - 126 of
this widget's 171 listener-attaching components - in a widget with only 251 DOM nodes. That
is a far worse components-per-node ratio than media-releases, which had 340 across 2,951
nodes.

Returning the day cell directly when disabled is behaviour-identical: a disabled HoverCard
never opens, so nothing observable is lost.

This is now the third place this exact Mantine behaviour has cost real work - after the
shared board context menu and the media-release tooltips. The general rule: Mantine overlay
components mount their full machinery regardless of `disabled`, so `disabled` is a
visual-suppression prop, not a cost-avoidance one. Deciding before rendering is what avoids
the cost.

packages/widgets/src/calendar/calender-day.tsx

@@ -22,6 +22,36 @@ export const CalendarDay = ({ date, events, disabled, rootHeight, rootWidth }: C
   const shouldScaleDown = minAxisSize < 350;
   const isSmall = rootHeight < 256;
 
+  const cell = (
+    <Container
+      h="100%"
+      w="100%"
+      p={0}
+      pt={isSmall ? 0 : 10}
+      pb={isSmall ? 0 : 10}
+      m={0}
+      pos="relative"
+      style={{
+        alignContent: "center",
+        borderRadius: actualItemRadius,
+        cursor: disabled ? "default" : "pointer",
+      }}
+    >
+      <Text ta={"center"} size={shouldScaleDown ? "xs" : "md"} lh={1}>
+        {date.getDate()}
+      </Text>
+      <NotificationIndicator events={events} isSmall={isSmall} />
+    </Container>
+  );
+
+  // Mantine mounts a HoverCard's Popover machinery — Popover, PopoverTarget and PopoverDropdown —
+  // even when it is disabled. A month grid is around 42 cells, and `disabled` is
+  // `isEditMode || eventsForDate.length === 0`, so on a typical month most days, and in edit mode
+  // every day, mounted three components for a card that can never open. Measured on a real board:
+  // 42 Popovers, 42 PopoverDropdowns and 42 PopoverTargets, 126 of this widget's 171
+  // listener-attaching components, in a widget with only 251 DOM nodes.
+  if (disabled) return cell;
+
   return (
     <HoverCard
       position="bottom"
@@ -32,29 +62,8 @@ export const CalendarDay = ({ date, events, disabled, rootHeight, rootWidth }: C
       transitionProps={{ transition: "pop" }}
       openDelay={350}
       closeDelay={400}
-      disabled={disabled}
     >
-      <HoverCard.Target>
-        <Container
-          h="100%"
-          w="100%"
-          p={0}
-          pt={isSmall ? 0 : 10}
-          pb={isSmall ? 0 : 10}
-          m={0}
-          pos="relative"
-          style={{
-            alignContent: "center",
-            borderRadius: actualItemRadius,
-            cursor: disabled ? "default" : "pointer",
-          }}
-        >
-          <Text ta={"center"} size={shouldScaleDown ? "xs" : "md"} lh={1}>
-            {date.getDate()}
-          </Text>
-          <NotificationIndicator events={events} isSmall={isSmall} />
-        </Container>
-      </HoverCard.Target>
+      <HoverCard.Target>{cell}</HoverCard.Target>
       <HoverCard.Dropdown maw="calc(100vw - 24px)" w={512} pe={4} pb={0} style={{ overflow: "hidden" }}>
         <CalendarEventList events={events} />
       </HoverCard.Dropdown>