Add result set column sorting (#34)

[muk2]

Summary

• Adds interactive column sorting in the results pane via the s key
• Cycles through: unsorted → ascending (▲) → descending (▼) → unsorted
• Sort indicators shown in column headers next to the sorted column name
• Supports type-aware comparison for all CellValue types (integers, floats, text, booleans, dates, timestamps) with NULLs always sorted last
• Cross-type numeric comparison (e.g. Int16 vs Int64, Int32 vs Float64)
• Sort state automatically resets when executing new queries or switching between result sets (Ctrl+[/])

Changes

• src/db/query.rs: Added sort_cmp() method on CellValue with comprehensive type-aware comparison logic and 7 unit tests
• src/ui/app.rs: Added toggle_column_sort(), clear_sort(), rebuild_sort_indices(), and sorted_row_index() methods; added sort state fields; wired s key handler
• src/ui/components.rs: Updated draw_result_table() to display sort indicators (▲/▼) in headers and render rows using sorted indices; added s shortcut to help overlay

Test plan

• All 223 tests pass (7 new sorting tests)
• cargo build compiles without warnings
• Manual: Open results pane, press s to sort ascending, press again for descending, press again to clear
• Manual: Switch result sets with Ctrl+[/] and verify sort resets
• Manual: Execute new query and verify sort resets

Closes #34

🤖 Generated with Claude Code