feat: add Notification Settings Modal and Stack Alert Sheet components

- Implemented NotificationSettingsModal for configuring notification agents (Discord, Slack, Webhook) and global settings.
- Added StackAlertSheet for managing stack-specific alert rules with metrics, thresholds, and cooldowns.
- Introduced reusable UI components: DropdownMenu, Popover, Select, Sheet, Slider, and Switch.
- Enhanced Tabs component by removing unnecessary client directive.

frontend/src/components/ui/popover.tsx

@@ -0,0 +1,31 @@
+import * as React from "react"
+import * as PopoverPrimitive from "@radix-ui/react-popover"
+
+import { cn } from "@/lib/utils"
+
+const Popover = PopoverPrimitive.Root
+
+const PopoverTrigger = PopoverPrimitive.Trigger
+
+const PopoverAnchor = PopoverPrimitive.Anchor
+
+const PopoverContent = React.forwardRef<
+  React.ElementRef<typeof PopoverPrimitive.Content>,
+  React.ComponentPropsWithoutRef<typeof PopoverPrimitive.Content>
+>(({ className, align = "center", sideOffset = 4, ...props }, ref) => (
+  <PopoverPrimitive.Portal>
+    <PopoverPrimitive.Content
+      ref={ref}
+      align={align}
+      sideOffset={sideOffset}
+      className={cn(
+        "z-50 w-72 rounded-md border bg-popover p-4 text-popover-foreground shadow-md outline-none data-[state=open]:animate-in data-[state=closed]:animate-out data-[state=closed]:fade-out-0 data-[state=open]:fade-in-0 data-[state=closed]:zoom-out-95 data-[state=open]:zoom-in-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 origin-[--radix-popover-content-transform-origin]",
+        className
+      )}
+      {...props}
+    />
+  </PopoverPrimitive.Portal>
+))
+PopoverContent.displayName = PopoverPrimitive.Content.displayName
+
+export { Popover, PopoverTrigger, PopoverContent, PopoverAnchor }