Fixes across client creation, Snapshots, and sign-in

Fixes

• Viewing a snapshot no longer registers as an edit. Opening a snapshot leaves its last-edited time untouched and no longer adds an empty placeholder entry.
• Explicit attach for existing people, and visible save errors. Adding or editing a client no longer silently combines two similar-looking people; reusing someone already in your book now goes through an explicit picker, and any blocking save error surfaces in the form footer and jumps to the field that needs attention.
• No onboarding flash on sign-in. Signing in with an already-onboarded account now goes straight to the dashboard, without the onboarding page briefly appearing in between.