Add option to block users on personal accounts from comments or PRs [GA]

### Value Prop
You can now block a user directly from a pull request or issue comment — no need to navigate away, visit a profile, or lose your place in the conversation. A streamlined modal lets you add an optional moderation note and confirm the action in just a few clicks, keeping your context intact. This makes it faster and less disruptive to handle spam and abuse right where it happens.

### Expected Outcome
Maintainers who deal with high volumes of spam or disruptive contributors should be able to act quickly without the overhead of a multi-step, multi-page workflow. By surfacing the block action in context and supporting moderation notes, we want to make it easier to maintain a healthy, focused contribution environment at scale. The goal is to give maintainers more control over their projects without pulling them away from the work that matters.

Start by reviewing the Value Prop and Expected Outcome, focusing on the comment and pull request contexts described. Define the scope for an in-context blocking modal, optional moderation notes, and confirmation flow; done means maintainers can block a user from either context without navigating away.

Written by the indexing model from the issue text.