feat: add Delta Lake connector + docs + test

[royendo]

Adds a Delta Lake connector that lets users query Delta tables stored on S3, Azure Blob Storage, or local filesystem, powered by DuckDB's delta extension.

• Backend: installs and loads the delta DuckDB extension at startup; adds project schema validation for Delta sources
• Frontend: multi-step form schema with storage type selector (S3 / Azure / local), conditional field visibility via x-grouped-fields, and disabled-option enforcement when cloud connectors are missing
• Source rewriting: maybeRewriteToDuckDb() normalizes Delta form data into a DuckDB model with delta_scan() SQL, bridging cloud credentials via create_secrets_from_connectors
• Icons: Delta Lake logo and icon Svelte components
• Docs: new Delta Lake connector page with storage backend matrix, UI walkthrough, and manual YAML configuration examples

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

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Developed in collaboration with Claude Code

[royendo]

@AdityaHegde , pulled changes from iceberg PR :)