[feat] JSON import MVP

[thoughtspile]

Hippotable should make working with widespread formats simple. The top priority at this moment is JSON.

• On "open file" click, the user should be able to select JSON files
• We should detect JSON / CSV format on the fly via file extension or mime — save it to FS API.
• At this stage, the only allowed JSON shape is array-of-objects. Show an error message if the schema does not match (related: #5 )

Additional context

In the future, it should be easy to connect to external APIs that serve JSON, but more work around response mapping, pagination, auth and CORS will be needed.