[management, reverse proxy] Add reverse proxy feature (#5291)

* implement reverse proxy


---------

Co-authored-by: Alisdair MacLeod <git@alisdairmacleod.co.uk>
Co-authored-by: mlsmaycon <mlsmaycon@gmail.com>
Co-authored-by: Eduard Gert <kontakt@eduardgert.de>
Co-authored-by: Viktor Liu <viktor@netbird.io>
Co-authored-by: Diego Noguês <diego.sure@gmail.com>
Co-authored-by: Diego Noguês <49420+diegocn@users.noreply.github.com>
Co-authored-by: Bethuel Mmbaga <bethuelmbaga12@gmail.com>
Co-authored-by: Zoltan Papp <zoltan.pmail@gmail.com>
Co-authored-by: Ashley Mensah <ashleyamo982@gmail.com>

proxy/web/src/data.ts

@@ -0,0 +1,54 @@
+// Auth method types matching Go
+export type AuthMethod = 'pin' | 'password' | 'oidc' | "link"
+
+// Page types
+export type PageType = 'auth' | 'error'
+
+// Error data structure
+export interface ErrorData {
+  code: number
+  title: string
+  message: string
+  proxy?: boolean
+  destination?: boolean
+  requestId?: string
+  simple?: boolean
+  retryUrl?: string
+}
+
+// Data injected by Go templates
+export interface Data {
+  page?: PageType
+  methods?: Partial<Record<AuthMethod, string>>
+  error?: ErrorData
+}
+
+declare global {
+  // eslint-disable-next-line no-var
+  var __DATA__: Data | undefined
+}
+
+export function getData(): Data {
+  const data = globalThis.__DATA__ ?? {}
+
+  // Dev mode: allow ?page=error query param to preview error page
+  if (import.meta.env.DEV) {
+    const params = new URLSearchParams(globalThis.location.search)
+    const page = params.get('page')
+    if (page === 'error') {
+      return {
+        ...data,
+        page: 'error',
+        error: data.error ?? {
+          code: 503,
+          title: 'Service Unavailable',
+          message: 'The service you are trying to access is temporarily unavailable. Please try again later.',
+          proxy: true,
+          destination: false,
+        },
+      }
+    }
+  }
+
+  return data
+}