chore: bump product to 0.6.0; multi-owner startup warning

Phase 3.5 ships → product minor bump under pre-1.0 rules (any surface
bump triggers minor). Schema is now 6 (0006_users_authz.sql); API
remains 1 (additive endpoints + new credential type, no breaking
shape changes). docs/versioning.md updated.

main.rs gets warn_on_multi_owner_install() which fires once after
bootstrap when more than one active owner exists — points the
operator at PATCH /admin/admins/{id} for cleanup. Soft-fail on DB
error (does not block startup).

The api-test schema assertion was updated to expect 6.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashboard/package.json

@@ -1,6 +1,6 @@
 {
   "name": "picloud-dashboard",
-  "version": "0.5.1",
+  "version": "0.6.0",
   "private": true,
   "type": "module",
   "scripts": {