Add BigQueryExplorer component and page

[marcocastignoli]

This PR implements the BigQuery component and page. There are two main parts in this PR:

• A BigQuery editor to execute queries: it calls a new API on server that uses our GCP service account to execute queries and display results to the user in a table
• A LLM SQL generator: the user can use natural language to ask questions about the Sourcify database. E.g. "Which contract is the most popular contract by contract name?"

The LLM SQL generator uses free models from Openrouter. Free usage is on our openrouter key that we embed in the browser. Users can use their own private key to use payed models.

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[kuzdogan]

Two things I can see here:

1. We should definitely make Examples a bit more prominent.
2. I think the BigQuery playground can be directly on the landing page. If not the whole playground, it should have a call to action and a button to go there, and it shouldn't occupy precious space on the header bar.
3. Can't we pull the available openrouter models to a dropdown instead of asking user to go to openrouter and copy-paste from there?
4. I still can't click "Generate SQL" with the default free model.

[marcocastignoli]

1. ok, good idea
2. ok, makes sense. Where would you put the call to action?
3. free models always change, so I thought it would be better to just manage them with a select. Also they are a lot. My idea is actually to hide the model selection even more
4. You cannot click it because there is no environment variable set yet

[kuzdogan]

How about we embed it directly after the verified contracts graph? Not even linking out to a separate page.

[kuzdogan]

I've done some layout changes and adding submission with ctrl+enter.

Just a few questions on the code.

But in general amazing work!

[marcocastignoli]

• add a notice that this is just a playground and it has limits
• point people to how they can use the "full version": bigquery and notebook