[User Onboarding] Add "Verify your phone number" modal after sign in

After signing into websites, sometimes they'll prompt you to...
- Double check your security settings
- Update your contact info (e.g. phone number, etc.)

Let's use this style of modal to prompt users to verify their phone number. We'll show this only after they sign in.

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I'll mention, I'm not completely in love with this UX because when you're signing into an app, you're doing so because you have a clear goal in mind. This style of UX adds a barrier and distraction to you completing that goal.

No files or tests are named. Start by locating the sign-in flow and any existing phone-verification or modal components, then confirm how authentication state is handled after sign-in. Done means the modal appears only after sign-in, prompts for phone verification, and does not incorrectly block other authentication paths.

Written by the indexing model from the issue text.