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.lock

@@ -161,6 +161,27 @@ dependencies = [
  "winapi",
 ]
 
+[[package]]
+name = "csv"
+version = "1.4.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "52cd9d68cf7efc6ddfaaee42e7288d3a99d613d4b50f76ce9827ae0c6e14f938"
+dependencies = [
+ "csv-core",
+ "itoa",
+ "ryu",
+ "serde_core",
+]
+
+[[package]]
+name = "csv-core"
+version = "0.1.13"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "704a3c26996a80471189265814dbc2c257598b96b8a7feae2d31ace646bb9782"
+dependencies = [
+ "memchr",
+]
+
 [[package]]
 name = "darling"
 version = "0.23.0"
@@ -374,6 +395,7 @@ dependencies = [
  "anyhow",
  "chrono",
  "crossterm",
+ "csv",
  "dirs",
  "flate2",
  "indexmap",
@@ -381,6 +403,7 @@ dependencies = [
  "ratatui",
  "serde",
  "serde_json",
+ "tempfile",
  "thiserror",
  "unicode-width 0.2.0",
 ]