feat(apps): make environment URLs prod-ready (env-driven, not hardcoded .local)

The apps were wired for the dev (.local) environment. Drive the base URLs from
env so one build serves dev and prod (.eu):

- portal nuxt.config: OIDC authorization/token/userinfo/discovery URLs +
  redirectUri now derive from NUXT_PUBLIC_AUTH_URL / NUXT_PUBLIC_PORTAL_URL
  (+ PORTAL_OIDC_APP_SLUG); .local defaults keep dev working with no env.
- portal sign-out handler: end-session + post-logout URLs env-driven.
- portal scheduling page: booking base/host from runtimeConfig.public.bookingUrl
  (NUXT_PUBLIC_BOOKING_URL).
- platform-api: tenant mail domain suffix from PLATFORM_TENANT_DOMAIN (dezky.eu
  in prod), defaulting to dezky.local.

(booking needs no change — its only .local ref is the dev-server allowedHosts.)

apps/portal/pages/admin/scheduling.vue

@@ -49,7 +49,8 @@ const toast = useToast()
 const { tenant } = useTenant()
 const slug = computed(() => tenant.value?.slug ?? '')
 const { request } = useApiFetch()
-const bookingBase = 'https://booking.dezky.local'
+const bookingBase = (useRuntimeConfig().public.bookingUrl as string) || 'https://booking.dezky.local'
+const bookingHost = bookingBase.replace(/^https?:\/\//, '')
 
 const base = computed(() => `/api/tenants/${slug.value}/scheduling`)
 
@@ -894,7 +895,7 @@ const maskSecret = (s: string) => (s.length > 12 ? `${s.slice(0, 9)}…${s.slice
               <template v-if="!hostForm.slug">Used in the public booking link.</template>
               <template v-else-if="!slugValid">Lowercase letters, numbers and hyphens (2–40 chars).</template>
               <template v-else-if="slugTaken">“{{ hostForm.slug }}” is already taken.</template>
-              <template v-else>booking.dezky.local/{{ slug }}/{{ hostForm.slug }} · available ✓</template>
+              <template v-else>{{ bookingHost }}/{{ slug }}/{{ hostForm.slug }} · available ✓</template>
             </span>
           </label>
           <label class="field"><Eyebrow>Timezone</Eyebrow>