feat(cabana): read-only Plugins screen with installed plugins

- GET /plugins (backend.view_plugins, superusers always pass) lists the
  plugins from party.Installed, or party.Activated IDs for older builds,
  with the built framework version and whether the registry is configured
- Settings -> System -> Plugins entry with the new SettingsEntry.path,
  added only for principals allowed to open it
- PluginsView at /system/plugins with an Installed tab (reka-ui Tabs);
  settings panel, rail, breadcrumbs and page title know the route
- OpenAPI and SPA types regenerated; contract, token-class and
  conformance inventories list the route; en/pl strings; docs

admin/src/views/PluginsView.vue

@@ -0,0 +1,126 @@
+<script setup lang="ts">
+import { onMounted, ref } from 'vue'
+import { TabsContent, TabsList, TabsRoot, TabsTrigger } from 'reka-ui'
+import { CircleAlert } from '@lucide/vue'
+import { api } from '../api/client'
+import type { InstalledPlugin, PluginsOverview } from '../api/types'
+import { t } from '../app/i18n'
+
+// The read-only Plugins screen (Settings → System, backend.view_plugins).
+// Installed lists the plugins the binary was built with, as summer build
+// baked them (GET /plugins). Every string from the server is rendered as
+// text, never as HTML.
+const overview = ref<PluginsOverview | null>(null)
+const failed = ref(false)
+const loading = ref(true)
+const tab = ref('installed')
+
+const SOURCE_LABELS: Record<string, string> = {
+  module: 'backend::lang.plugins.source_module',
+  submodule: 'backend::lang.plugins.source_submodule',
+  local: 'backend::lang.plugins.source_local',
+}
+
+function sourceLabel(plugin: InstalledPlugin): string {
+  const key = SOURCE_LABELS[plugin.source]
+  return key ? t(key) : plugin.source
+}
+
+async function load(): Promise<void> {
+  loading.value = true
+  failed.value = false
+  try {
+    const { data, response } = await api.GET('/plugins')
+    if (response.ok && data) {
+      overview.value = data.data
+    } else {
+      failed.value = true
+    }
+  } catch {
+    failed.value = true
+  } finally {
+    loading.value = false
+  }
+}
+
+onMounted(load)
+
+const tabClass =
+  'inline-flex h-[34px] items-center gap-2 rounded-tab px-4 font-semibold text-muted transition-colors duration-150 ease-out hover:text-text data-[state=active]:bg-surface data-[state=active]:text-text data-[state=active]:shadow-tab'
+</script>
+
+<template>
+  <section class="flex flex-col gap-5" data-plugins-screen>
+    <header class="flex flex-col gap-1">
+      <h1 class="text-[26px] font-bold tracking-[-0.02em]">{{ t('backend::lang.plugins.title') }}</h1>
+      <p v-if="overview?.framework" class="text-[13px] text-muted" data-plugins-framework>
+        {{ t('backend::lang.plugins.framework', { version: overview.framework }) }}
+      </p>
+    </header>
+
+    <p v-if="failed" role="alert" class="flex items-center gap-2 text-danger" data-plugins-failed>
+      <CircleAlert :size="16" class="shrink-0" aria-hidden="true" />
+      <span>{{ t('backend::lang.plugins.load_failed') }}</span>
+    </p>
+
+    <TabsRoot v-else-if="overview" v-model="tab" class="flex flex-col gap-4">
+      <TabsList
+        :aria-label="t('backend::lang.plugins.tabs_label')"
+        class="inline-flex gap-1 self-start rounded-inner border border-border bg-subtle p-1"
+      >
+        <TabsTrigger value="installed" data-tab="installed" :class="tabClass">
+          {{ t('backend::lang.plugins.tab_installed') }}
+        </TabsTrigger>
+        <TabsTrigger v-if="overview.registry.enabled" value="marketplace" data-tab="marketplace" :class="tabClass">
+          {{ t('backend::lang.plugins.tab_marketplace') }}
+        </TabsTrigger>
+      </TabsList>
+
+      <TabsContent value="installed" class="flex flex-col gap-3">
+        <p v-if="overview.plugins.length === 0" class="text-muted">{{ t('backend::lang.plugins.empty') }}</p>
+        <div v-else class="overflow-x-auto rounded-card border border-border bg-surface shadow-card">
+          <table class="w-full border-collapse text-left">
+            <thead>
+              <tr class="border-b border-border text-[13px] text-muted">
+                <th scope="col" class="px-5 py-3 font-semibold">{{ t('backend::lang.plugins.column_plugin') }}</th>
+                <th scope="col" class="px-5 py-3 font-semibold">{{ t('backend::lang.plugins.column_version') }}</th>
+                <th scope="col" class="px-5 py-3 font-semibold">{{ t('backend::lang.plugins.column_source') }}</th>
+              </tr>
+            </thead>
+            <tbody>
+              <tr
+                v-for="plugin in overview.plugins"
+                :key="plugin.id"
+                :data-plugin="plugin.id"
+                class="border-b border-border align-top last:border-b-0"
+              >
+                <td class="px-5 py-3">
+                  <div class="flex min-w-0 flex-col">
+                    <span class="font-semibold" data-plugin-name>{{ plugin.name || plugin.id }}</span>
+                    <span v-if="plugin.name" class="font-mono text-[12px] text-muted">{{ plugin.id }}</span>
+                    <span v-if="plugin.description" class="text-[13px] text-muted" data-plugin-description>{{
+                      plugin.description
+                    }}</span>
+                  </div>
+                </td>
+                <td class="px-5 py-3 whitespace-nowrap">
+                  <span v-if="plugin.version" class="font-mono text-[13px]" data-plugin-version>{{ plugin.version }}</span>
+                  <span v-else class="text-[13px] text-muted" data-plugin-version>{{
+                    t('backend::lang.plugins.no_version')
+                  }}</span>
+                </td>
+                <td class="px-5 py-3 text-[13px] whitespace-nowrap" data-plugin-source>
+                  {{ plugin.source ? sourceLabel(plugin) : '' }}
+                </td>
+              </tr>
+            </tbody>
+          </table>
+        </div>
+      </TabsContent>
+
+      <TabsContent v-if="overview.registry.enabled" value="marketplace" class="flex flex-col gap-4" />
+    </TabsRoot>
+
+    <p v-else-if="loading" class="text-muted" aria-busy="true">…</p>
+  </section>
+</template>