docs: add Google Sheets integration design catalog

[BorisTyshkevich]

Summary

Add a version-controlled design catalog for the browser-only Google Sheets integration.

The catalog defines:

• product scope and ownership boundaries;
• browser-only architecture and authentication;
• versioned binding and checkpoint contracts;
• managed spreadsheet layout;
• snapshot export;
• failure-safe full refresh;
• ClickHouse 26.8+ append refresh using per-partition _block_number / _block_offset checkpoints;
• large-result streaming and Google quota handling;
• recovery, sharing, security, configuration, and testing;
• accepted architecture decisions;
• three self-contained HTML companion diagrams.

Key decisions

• Repository design docs are the source of truth; issues are executable implementation slices.
• Google never calls SQL Browser. Manual refresh launches the ClickHouse-hosted SPA through a hyperlink.
• Bindings are append-only versions stored in a configured ClickHouse table.
• Full refresh stages before publishing into the stable managed sheet ID.
• Append refresh is available only for validated ClickHouse 26.8+ MergeTree-family sources.
• Append uses finite queries and per-partition _block_number / _block_offset; SELECT ... STREAM, updates, and deletes are out of scope.
• No silent truncation or silent fallback from append to full refresh.

Review entry point

docs/design/google-sheets/README.md

Scope

Documentation only. No runtime behavior changes.