Add invitingOrganizations field on viewer

Expose viewer.invitingOrganizations: [Organization!]! returning the
organizations that have a live pending invitation directed at the
current identity (accepted_at IS NULL AND expires_at > NOW()). The
list is rendered under a "Pending invitations" section on the
memberships page and in the organization selector dropdown, so a user
already signed in with an existing identity can see which
organizations have invited them without having to dig through their
inbox.

The new field is gated by iam:invitation:list against the viewer's
own identity, so it does not loosen authorization on Organization
elsewhere. E2E coverage validates the live-pending case, the
no-invitation and post-accept cases, and a multi-org scenario.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

e2e/internal/testutil/client.go

@@ -530,6 +530,10 @@ func NewClientWithNewSession(t testing.TB, from *Client) *Client {
 	return client
 }
 
+func (c *Client) GetEmail() string {
+	return c.email
+}
+
 func (c *Client) GetUserID() gid.GID {
 	return c.userID
 }