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.

Cargo.toml

@@ -21,9 +21,11 @@ chrono = { version = "0.4", features = ["serde"] }
 flate2 = "1"
 unicode-width = "0.2"
 dirs = "5"
+csv = "1"
 
 [dev-dependencies]
 proptest = "1"
+tempfile = "3"
 
 [profile.release]
 opt-level = 3