feat: export query results to CSV, JSON, SQL INSERT, and TSV

[muk2]

Summary

Adds multi-format export for query results, addressing #13.

• Ctrl+S in results view opens export format picker
• 5 export formats: CSV, JSON, SQL INSERT, TSV, clipboard copy
• Format picker dialog — navigate with arrow keys, quick-select with 1-5, or press Enter
• Timestamped filenames — pgrsql_export_YYYYMMDD_HHMMSS.{ext}
• Proper escaping — CSV quoting, SQL single-quote escaping, JSON type mapping
• NULL handling — empty in CSV, null in JSON, NULL in SQL

Export Formats

Format	Behavior
CSV	RFC 4180 compliant with proper quoting
JSON	Pretty-printed array of objects with correct types
SQL INSERT	Individual INSERT statements with proper escaping
TSV	Tab-separated with tab chars replaced in values
Clipboard	CSV format copied to system clipboard

Changes

File	Change
src/export.rs	New module: to_csv(), to_json(), to_sql_insert(), to_tsv() with helpers
src/main.rs	Added mod export
src/ui/app.rs	Added ExportFormat enum, Focus::ExportPicker, handle_export_input(), perform_export()
src/ui/components.rs	Added draw_export_picker() dialog, updated help overlay

Test plan

• All 133 tests pass (9 new export tests)
• Clippy clean
• cargo fmt clean
• Verify Ctrl+S opens format picker from results
• Verify 1-5 quick-select exports in correct format
• Verify CSV output has proper quoting/escaping
• Verify JSON output has correct types (numbers, booleans, nulls)
• Verify SQL INSERT output has proper single-quote escaping
• Verify clipboard copy works
• Verify file is saved with correct extension

Closes #13

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