feat(operator): avatar dropdown context menu in topbar

New UserMenu component owns its own trigger + dropdown + dismissal so the
topbar stays simple. Menu contents: identity row (name + email), theme
toggle (reuses useTweaks so the floating panel and menu stay in sync),
link to /settings, Sign out (calls useOidcAuth().logout).

Dismissal: outside click via a transparent Teleport scrim, Escape, and
route change (watch on route.path → close).

Drops the now-unused useOidcAuth import from OpTopbar.

apps/operator/components/OpTopbar.vue

@@ -1,7 +1,6 @@
 <script setup lang="ts">
 withDefaults(defineProps<{ oncall?: boolean }>(), { oncall: true })
 
-const { user } = useOidcAuth()
 const { state: tweaks } = useTweaks()
 const { open: openPalette } = useCommandPalette()
 
@@ -39,7 +38,7 @@ const env = computed(() => tweaks.value.env)
       <span class="icon-btn-dot" />
     </button>
 
-    <Avatar :name="user?.userInfo?.name || user?.userName || '?'" :size="26" />
+    <UserMenu />
   </header>
 </template>