Add tab completion for table and column names

[rohitgs28]

Summary

When typing SQL in the query input, pressing Tab should show a popup with matching table/column names from the schema.

Current Behavior

Tab cycles between panels.

Expected Behavior

• When the cursor is in the query input and text is partially typed, Tab shows completions
• Completions are sourced from app.schema.tables and their columns
• Arrow keys navigate the popup, Enter selects
• Tab without partial text still cycles panels

Technical Notes

• Schema is already available in App.schema: SchemaInfo
• Consider fuzzy matching (e.g. us matches users)
• Popup can be rendered as a floating ratatui widget
• See src/app.rs handle_key() for input handling

Files to modify

• src/app.rs (Tab key handling)
• src/ui/renderer.rs (completion popup rendering)
• NEW: src/ui/components/autocomplete.rs