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/ShopfloorDashboard.vue

@@ -444,7 +444,7 @@ function handlePhotoError(e) {
 .location-title {
   font-size: 18px;
   font-weight: 600;
-  color: #888;
+  color: #b8c4d8;
   text-transform: uppercase;
   letter-spacing: 2px;
 }
@@ -452,6 +452,7 @@ function handlePhotoError(e) {
 .header-center h1 {
   font-size: 28px;
   font-weight: 700;
+  color: #ffffff;
   text-transform: uppercase;
   letter-spacing: 2px;
   margin: 0;