Support reporting a message

[alya]

In zulip/zulip#34618, we added an option to report a message to moderators from the web app. We should support this feature on mobile as well.

The feature should be accessible from the message long-press menu. It's fine for the initial version not to show a preview of the message being reported.

Blockers

• Add supported moderation categories to API zulip#36489

[lakshay909]

Hi! @gnprice 👋 I’d love to work on this issue — thank you for maintaining such a well-structured project. I’m a student developer experienced in Flutter and API integrations, with prior contributions to open-source projects focused on improving UI performance and code consistency.

My plan is to first reproduce the issue locally, analyze the affected component behavior, and then implement a fix ensuring consistency with existing state management and theming patterns. I’ll validate the solution through widget tests and a manual QA pass on both Android and iOS. I can open a PR within 3–5 days after assignment.

Kindly assign me this issue — I’d be happy to contribute a reliable and maintainable fix. 🚀