feat: share credentials with users and roles, inherit data on account deletion (#1342)

* feat: share credentials with users and roles, inherit data on account deletion

Credentials can be shared at "use" or "manage" level. Recipients get
a copy re-encrypted under their own data key (shared_credential_secrets),
kept in step with the owner's row through the same lifecycle hooks as
shared host secrets. One gate, findUsableCredential(), replaces the
private-namespace lookups so a shared credential works wherever a
private one does. Deleting a user now hands their hosts and credentials
to a successor (the deleting admin by default) instead of revoking
everything they shared.

* fix: harden credential ownership transfer

src/ui/tests/sidebar/AdminUserManagePanel.test.tsx

@@ -24,6 +24,7 @@ const api = vi.hoisted(() => ({
   createApiKey: vi.fn(async () => ({})),
   deleteApiKey: vi.fn(async () => ({})),
   deleteUser: vi.fn(async () => ({})),
+  getUserList: vi.fn(async () => ({ users: [] })),
   getUserRoles: vi.fn(async () => ({ roles: [] as unknown[] })),
   assignRoleToUser: vi.fn(async () => ({})),
   removeRoleFromUser: vi.fn(async () => ({})),