tower-ui 0.61.20 — Migrate picker hides platform server

MigrateDrawer hardcoded '<option value="">Platform server (default)</option>' as
the first picker entry, regardless of tenant scope. A tenant operator
saw it as a selectable default — and selecting it (or just leaving
the default empty) sent the migrate to the platform cluster, which
the operator has no business deploying to.

Now: removed the hardcoded option. Auto-pick the first deployable
non-platform server on load (matches DeployInstanceDrawer pattern).
Picker shows 'Pick a server…' as a disabled placeholder when nothing
is selected.

values.yaml

@@ -29,7 +29,7 @@ frontend:
   enabled: true
   image:
     repository: registry.odoosky.cloud/odoosky/docker-mirror/tower-ui
-    tag: "0.61.19"
+    tag: "0.61.20"
     pullPolicy: IfNotPresent
   imagePullSecrets:
     - name: docker-mirror-pull