Add four developer/QA features:
- Connection color coding: color picker in dialog, colored indicators across toolbar, tabs, status bar, and connection selectors
- Query history: Rust backend with JSON file storage (500 entry cap), sidebar panel with search/clear, auto-recording from workspace
- Schema-aware SQL autocomplete: backend fetches column metadata, CodeMirror receives schema namespace with public tables unprefixed
- EXPLAIN ANALYZE visualizer: recursive tree view with cost-colored bars, expand/collapse nodes, buffers info, Results/Explain tab toggle
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
