Code Insights: Add 404 screen for the standalone insight page

Created by: vovakulikov

Closes https://github.com/sourcegraph/sourcegraph/issues/35045

Test plan

• Make sure that we handle not found insight properly on the standalone insight page
• Make sure that other fetch logic doesn't have regressions

@AlicjaSuska I think we still need to discuss the original dashboard like 404 page. In this PR I implemented the first version where we have only 404 screen without additional dashboard-like UI (select, action buttons, ...etc)

Also can you please add colors for the chart for our dark mode color schema

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.