Clearer errors when a shared account login fails

When Handhold cannot sign in to a shared account, the product access page now tells you what stopped it and what to do about it.

• Fixes you can make yourself are spelled out. Rejected credentials, a temporary password that must be changed, a verification step the method is not set up for, an unverified email, a locked account, a pending onboarding step, a missing login email, a bot check we cannot pass, or instructions that no longer match the login page each get a specific message.
• Problems on our side are flagged as such. If the failure is a technical one, the message says so and the Handhold team is notified automatically.
• Failures stay visible. A method that stops working between tests shows "Needs attention" on the product access page, with the reason, until you test it again and the login succeeds.