feat(auth): OIDC login UI + binding management + OIDC-only mode

- useAuthConfig fetches public /auth/config; LoginPage hides the
  password form when oidc_only and shows an SSO button when enabled.
- /oidc/callback route applies the returned JWT (sign-in) or shows the
  link result; oidc_error surfaced on LoginPage.
- UsersPage: hides password fields in OIDC-only mode; admin OIDC
  bind/unbind UI per user. Sidebar self-service "Link OIDC account"
  (non-OIDC_ONLY).
- Dockerfile ARG/ENV HARBORFORGE_OIDC_ONLY.

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

src/hooks/useAuthConfig.ts

@@ -0,0 +1,76 @@
+import { useState, useEffect } from 'react'
+import api from '@/services/api'
+
+export interface AuthConfig {
+  oidcEnabled: boolean
+  oidcOnly: boolean
+  passwordLogin: boolean
+  oidcLoginUrl: string
+}
+
+const DEFAULT: AuthConfig = {
+  oidcEnabled: false,
+  oidcOnly: false,
+  passwordLogin: true,
+  oidcLoginUrl: '/auth/oidc/login',
+}
+
+let cache: AuthConfig | null = null
+let inflight: Promise<AuthConfig> | null = null
+
+async function load(): Promise<AuthConfig> {
+  if (cache) return cache
+  if (inflight) return inflight
+  inflight = api
+    .get('/auth/config')
+    .then(({ data }) => {
+      cache = {
+        oidcEnabled: !!data.oidc_enabled,
+        oidcOnly: !!data.oidc_only,
+        passwordLogin: data.password_login !== false,
+        oidcLoginUrl: data.oidc_login_url || '/auth/oidc/login',
+      }
+      return cache
+    })
+    .catch(() => {
+      // Backend unreachable / old backend without /auth/config:
+      // fall back to password-only so login is never fully blocked.
+      cache = { ...DEFAULT }
+      return cache
+    })
+    .finally(() => {
+      inflight = null
+    })
+  return inflight
+}
+
+/** Absolute backend URL for full-page OIDC redirects. */
+export function oidcLoginHref(cfg: AuthConfig): string {
+  const base = localStorage.getItem('HF_BACKEND_BASE_URL') ?? ''
+  return `${base}${cfg.oidcLoginUrl}`
+}
+
+export function oidcLinkHref(): string {
+  const base = localStorage.getItem('HF_BACKEND_BASE_URL') ?? ''
+  return `${base}/auth/oidc/link`
+}
+
+export function useAuthConfig() {
+  const [config, setConfig] = useState<AuthConfig | null>(cache)
+  const [loading, setLoading] = useState(!cache)
+
+  useEffect(() => {
+    let alive = true
+    load().then((c) => {
+      if (alive) {
+        setConfig(c)
+        setLoading(false)
+      }
+    })
+    return () => {
+      alive = false
+    }
+  }, [])
+
+  return { config: config ?? DEFAULT, loading }
+}