Data source: BigQuery plugin

Google Cloud data warehouse. **SQL over HTTP REST**.

Builds on the **Seshat** foundation (#93, shipped in #99): host `tcp-client` (+ host-side TLS), owner-thread query execution, `secure-storage` for credentials, and the `UiNode` DSL (connection dialog, dock tabs, results grid, `json-tree`).

## Transport
- BigQuery REST (`jobs.query`) over HTTPS `:443`. Auth: service-account JSON → OAuth2 access token (JWT-signed token exchange). Config: project, dataset, location.

## Reuse
- `DbAdapter`, results grid.

## New work
- HTTPS client; OAuth2 service-account token minting; BigQuery REST query + schema (`datasets`, `tables`) calls; project/dataset config.

Depends on the shared **host HTTP client** prerequisite (see umbrella).

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Part of the data-source plugins epic #109.