Image annotation (areas, rectangles)

## Feature Request Form
Feature request from Zendesk: https://hypothesis.zendesk.com/agent/tickets/2563

Image annotation is mentioned in https://github.com/hypothesis/vision/issues/51, but not anywhere in our backlog that I can find.

### Problem you are trying to address with this feature
Currently annotations can only anchor to lines of text or to pages (as Page Notes). There is no way to annotate images or areas of pages.

### Your solution
User has requested the ability to highlight an area of a page / document by creating a rectangle or box. There are other ways to implement, but this is how this user would expect the feature to work.

Start by reading the linked vision issue #51 and the referenced Zendesk ticket to understand the requested image and page-area annotation behavior. Done means agreeing on an implementation approach and supporting rectangle or box annotations, but this issue does not identify files, tests, or a concrete entry point.

Written by the indexing model from the issue text.