feat: add React Flow UI foundation components for network diagrams

BaseNode (structured node shell with header/content/footer slots),
BaseHandle (styled connection handle), LabeledHandle (handle with
port label), NodeStatusIndicator (status border effect),
NodeTooltip (hover details via NodeToolbar).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/network/ui/base-node.tsx

@@ -0,0 +1,56 @@
+import type { ComponentProps } from 'react'
+import { cn } from '@/lib/utils'
+
+export function BaseNode({ className, ...props }: ComponentProps<'div'>) {
+  return (
+    <div
+      className={cn(
+        'bg-card text-heading relative rounded-lg border border-default',
+        'transition-colors hover:border-hover',
+        'in-[.selected]:border-accent',
+        className,
+      )}
+      tabIndex={0}
+      {...props}
+    />
+  )
+}
+
+export function BaseNodeHeader({ className, ...props }: ComponentProps<'header'>) {
+  return (
+    <header
+      {...props}
+      className={cn('flex flex-row items-center gap-2 px-3 py-2', className)}
+    />
+  )
+}
+
+export function BaseNodeHeaderTitle({ className, ...props }: ComponentProps<'h3'>) {
+  return (
+    <h3
+      data-slot="base-node-title"
+      className={cn('select-none flex-1 text-xs font-semibold text-heading', className)}
+      {...props}
+    />
+  )
+}
+
+export function BaseNodeContent({ className, ...props }: ComponentProps<'div'>) {
+  return (
+    <div
+      data-slot="base-node-content"
+      className={cn('flex flex-col gap-y-1 px-3 pb-2', className)}
+      {...props}
+    />
+  )
+}
+
+export function BaseNodeFooter({ className, ...props }: ComponentProps<'div'>) {
+  return (
+    <div
+      data-slot="base-node-footer"
+      className={cn('flex flex-col items-center gap-y-1 border-t border-default px-3 pt-1.5 pb-2', className)}
+      {...props}
+    />
+  )
+}