Do not show "Thanks for signing up" flash to existing members completing their profile

[mroderick]

Problem

The "Thanks for signing up" flash (notifications.signing_up, shown in
Member::DetailsController#edit) is misleading for returning members. It reads
as if a brand-new (duplicate) account was created, when in fact the member was
simply routed to the "Almost there..." details page because their profile is
incomplete.

This is a residual of the 28 Jul /auth/codebar feedback and remains after
#2785 (which gates the details redirect on
member.requires_additional_details?). A complete member no longer hits this
path, but an incomplete existing member (missing any of
name/surname/about_you) still does — and still gets told "Thanks for signing
up", which is wrong for a returning account.

Proposed fix

Only flash signing_up when the member was actually created during the OAuth
callback, not when an existing member is sent to the details page.

In AuthServicesController#create, the else branch already tracks whether the
member was created (member.save! succeeded on a new record). Pass that signal
through so a find-existing redirect doesn't set the signup flash. The details
controller should show "Thanks for signing up" / "Almost there..." only for
genuinely new members; existing members should get a neutral message (e.g.
"Please complete your profile") or none.

Acceptance criteria

• An existing, incomplete member logging in via a new provider is sent to
  the details page without the "Thanks for signing up" flash.
• A brand-new member still sees the "Thanks for signing up" flash.
• Specs cover both cases (extend auth_services_callback_spec.rb / the
  details feature spec).