Add CSV import functionality

- Use csv crate for robust CSV parsing (handles quoted fields, empty values, \r\n)
- Extend --import command to auto-detect format by file extension (.csv or .json)
- Reuse existing ImportPipeline and analyzer for field type detection
- Categories detected automatically (string fields), measures for numeric fields
- Updated help text and welcome screen to mention CSV support

All 201 tests pass.

src/import/mod.rs

@@ -1,2 +1,3 @@
 pub mod analyzer;
+pub mod csv_parser;
 pub mod wizard;