UI redesign: Ask, Explore, SQL, System tabs

- Ask: natural language → AI generates SQL → DataFusion executes → results
  Shows the AI-over-data-lake story: schema introspection → LLM → query
- Explore: click dataset → schema + preview + AI-generated summary
- SQL: raw DataFusion SQL editor with Ctrl+Enter
- System: health grid testing all 5 services + embeddings + generation
- Example prompts for quick demo
- Dark theme with accent styling

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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+}