Add AI SQL Query Assistant for database-sourced charts

Adds a natural-language SQL assistant panel to the Source → Import from
database tab of the chart editor. Users describe the data they want in
plain English and the AI generates a correctly structured, ready-to-run
SQL query tailored to the active chart type and database schema.

Key features:
- Multi-provider support: OpenAI, Gemini, Claude (panel only renders
  when at least one API key is configured)
- Chart-type-aware query generation (correct column count/types per chart)
- Table-aware: passes available tables and columns to the AI
- Conversational refinement: follow-up prompts modify the existing query
  rather than starting fresh, using the last generated query as context
- MySQL compatibility layer: detects environments where applying date
  functions to columns in WHERE crashes the connection (e.g. Local by
  Flywheel) and rewrites generated queries to use safe direct comparisons
- Empty-result detection: when a valid WHERE clause returns 0 rows and
  MySQL drops the connection, pings the DB to distinguish this from a
  real outage and shows a friendly 'No results found' message instead
- Suggestion chips for related query ideas
- One-click 'Use This Query' button injects generated SQL into CodeMirror

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: vytisbulkevicius