feat(ST-1109.6): add Popover primitive (base-ui wrapper)

- Mirrors the Tooltip pattern: render-prop composition, data-slot attrs
- Exports Popover (Root), PopoverTrigger, PopoverContent (Portal+Positioner+Popup)
- MD3 popover/popover-foreground tokens, animated open/close states
- Will be used to consolidate the backlog filter UI in ST-1109.8

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

components/ui/popover.tsx

@@ -0,0 +1,52 @@
+"use client"
+
+import { Popover as PopoverPrimitive } from "@base-ui/react/popover"
+
+import { cn } from "@/lib/utils"
+
+function Popover({ ...props }: PopoverPrimitive.Root.Props) {
+  return <PopoverPrimitive.Root data-slot="popover" {...props} />
+}
+
+function PopoverTrigger({ ...props }: PopoverPrimitive.Trigger.Props) {
+  return <PopoverPrimitive.Trigger data-slot="popover-trigger" {...props} />
+}
+
+function PopoverContent({
+  className,
+  side = "bottom",
+  sideOffset = 8,
+  align = "center",
+  alignOffset = 0,
+  children,
+  ...props
+}: PopoverPrimitive.Popup.Props &
+  Pick<
+    PopoverPrimitive.Positioner.Props,
+    "align" | "alignOffset" | "side" | "sideOffset"
+  >) {
+  return (
+    <PopoverPrimitive.Portal>
+      <PopoverPrimitive.Positioner
+        align={align}
+        alignOffset={alignOffset}
+        side={side}
+        sideOffset={sideOffset}
+        className="isolate z-50"
+      >
+        <PopoverPrimitive.Popup
+          data-slot="popover-content"
+          className={cn(
+            "z-50 w-72 origin-(--transform-origin) rounded-md border bg-popover p-4 text-popover-foreground shadow-md outline-none data-open:animate-in data-open:fade-in-0 data-open:zoom-in-95 data-closed:animate-out data-closed:fade-out-0 data-closed:zoom-out-95 data-[side=bottom]:slide-in-from-top-2 data-[side=left]:slide-in-from-right-2 data-[side=right]:slide-in-from-left-2 data-[side=top]:slide-in-from-bottom-2",
+            className
+          )}
+          {...props}
+        >
+          {children}
+        </PopoverPrimitive.Popup>
+      </PopoverPrimitive.Positioner>
+    </PopoverPrimitive.Portal>
+  )
+}
+
+export { Popover, PopoverTrigger, PopoverContent }