feat: interactive data table UX — sorting, copy, EXPLAIN, column reorder

### What does this issue track?

Follow-up to PR #141 (data-focused UX polish). The initial PR shipped rich query tables, progress bars, cost badges, schema previews, type indicators, percentage bars, relative timestamps, column statistics, and boolean/NULL styling.

This issue tracks the **interactive TUI features** that require state management and keyboard handling — higher complexity, separate scope.

### Proposed features

1. **Interactive sorting** — keyboard shortcuts to sort result table by column (▲/▼ indicators in header, frozen headers while scrolling)
2. **Copy-to-clipboard** — select cell/row/column and copy as CSV, JSON, or SQL `IN` clause (`C`/`J`/`L` shortcuts)
3. **EXPLAIN plan visualization** — auto-run `EXPLAIN ANALYZE`, render as ASCII tree with cost indicators and index usage hints (per-dialect: PostgreSQL, Snowflake, BigQuery, Databricks)
4. **Column reorder + hide/show** — `←`/`→` to reorder, `H` to toggle visibility, persist to session state, show "+ N hidden columns" indicator

### Why

These directly address common data engineering workflows:
- **Sorting**: avoid rewriting `ORDER BY` and re-executing queries during exploration
- **Copy**: paste IDs into `WHERE IN`, copy metrics for reports, copy column names for `SELECT` lists
- **EXPLAIN**: instant performance diagnosis without leaving the tool
- **Column reorder**: wide result sets (20+ cols) become scannable by hiding noise

### Research source

Full research in memory: competitive analysis of DataGrip, DBeaver, Snowsight, k9s, lazygit patterns. See PR #141 discussion for screenshots of the foundation these build on.

### Implementation notes

- All features require interactive TUI state (keyboard event handlers, re-render on state change)
- Sorting and column reorder can share a column state management layer
- EXPLAIN is the highest complexity — requires per-dialect SQL parsing (~200 LOC per DB)
- Copy needs Node.js clipboard integration
- Each feature can be shipped independently

### Checklist

- [ ] Interactive sorting with header indicators
- [ ] Copy-to-clipboard (cell, row, column, SQL formats)
- [ ] EXPLAIN plan ASCII tree visualization
- [ ] Column reorder + hide/show with session persistence