Add platform-dash: S3 artifact browser for Garage object store

Lightweight Next.js app for browsing, uploading, and downloading
artifacts from the cluster-local Garage S3 bucket. Uses the harness
design system. Features:
- File/folder browser with breadcrumb navigation
- Drag-and-drop upload
- Download and delete
- Ingress at platform.coreworlds.io (internal-only)

Also adds platform-dash to CI/deploy workflows.

infra/kubernetes/platform/traefik/certificate-internal.yaml

@@ -75,3 +75,16 @@ spec:
     kind: ClusterIssuer
   dnsNames:
     - s3.coreworlds.io
+---
+apiVersion: cert-manager.io/v1
+kind: Certificate
+metadata:
+  name: platform-dash-tls
+  namespace: platform
+spec:
+  secretName: platform-dash-tls
+  issuerRef:
+    name: letsencrypt-production
+    kind: ClusterIssuer
+  dnsNames:
+    - platform.coreworlds.io