Add inbound country flags and node load in location picker

- Inbound.CountryCode (ISO alpha-2), set on publish; flag shown before
  the location name (SVG via country-flag-icons on the site, emoji in bot)
- Node.Capacity (optional, not a limit); available inbounds report load
  percent of capacity or active config count, sorted least loaded first

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/shared/ui/country-flag.tsx

@@ -0,0 +1,11 @@
+import { FLAGS } from '@/shared/lib/countries'
+import { cn } from '@/shared/lib/cn'
+
+export function CountryFlag({ code, className }: { code: string | null | undefined; className?: string }) {
+  if (!code) return null
+  const Flag = FLAGS[code.toUpperCase()]
+  if (!Flag) {
+    return <span className={cn('font-mono text-xs text-muted-foreground', className)}>{code.toUpperCase()}</span>
+  }
+  return <Flag className={cn('inline-block h-3.5 w-auto shrink-0 rounded-[2px]', className)} aria-hidden />
+}