Fix PC installed-applications rendering; minor UI cleanups

The PC detail Installed Applications section 500d and vanished on any
real PC: ComputerInstalledApp had no to_dict, so the endpoint errored
and the v-if hid the section. Added the serializer (curated version
wins over the raw collected string, app name + description included)
and aligned PCDetail to the flat payload; regression test added.

Also: employee detail skips its USB panels when the usb plugin is
disabled (was firing 404s), and the shopfloor kiosk header is now
light-on-dark for readability.

810 tests pass; PC 259 installed apps verified live.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/views/pcs/PCDetail.vue

@@ -191,15 +191,15 @@
               <router-link
                 v-for="app in installedApps"
                 :key="app.id"
-                :to="`/applications/${app.application?.appid}`"
+                :to="`/applications/${app.appid}`"
                 class="app-item"
               >
                 <div class="app-info">
-                  <span class="app-name">{{ app.application?.appname }}</span>
-                  <span class="app-version" v-if="app.version">v{{ app.version }}</span>
+                  <span class="app-name">{{ app.appname }}</span>
+                  <span class="app-version" v-if="app.installedversion">v{{ app.installedversion }}</span>
                 </div>
-                <div class="app-desc" v-if="app.application?.appdescription">
-                  {{ app.application.appdescription }}
+                <div class="app-desc" v-if="app.appdescription">
+                  {{ app.appdescription }}
                 </div>
               </router-link>
             </div>