Data source: Snowflake plugin

Cloud data warehouse. **SQL over HTTP REST** (no raw socket protocol).

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
- Snowflake SQL API v2 over HTTPS `:443` (async: submit statement, poll for results). Auth: key-pair **JWT** (or OAuth). Config: account, warehouse, role, database/schema.

## Reuse
- `DbAdapter`, results grid.

## New work
- HTTPS client; SQL REST API submit/poll; JWT key-pair auth (RSA signing — we already pull in `rsa` for MySQL); account/warehouse/role config fields.

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

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Part of the data-source plugins epic #109.