feat(export): CSV export for the bet journal + forward-test ledger

Adds CSV export alongside the Excel snapshot export: two buttons on the Export hub
write the bet journal and the paper-trading ledger to UTF-8 (BOM) .csv files in the
configured export directory and toast the path — mirroring the Excel use case's
write-and-return-path contract. CSV needs no third-party library, so it lives in the
Application layer behind a pure RFC-4180 formatter.

- Csv formatter (RFC 4180 escaping) + ExportToCsvUseCase (journal + ledger, batched
  title join, returns null when there's nothing to export); registered; Export hub
  buttons + en/ru resx.
- 11 tests: formatter escaping/Document + use-case empty-state + real-file write to a temp dir.

tests/Marathon.Application.Tests/Export/CsvTests.cs

@@ -0,0 +1,41 @@
+using FluentAssertions;
+using Marathon.Application.Export;
+
+namespace Marathon.Application.Tests.Export;
+
+public sealed class CsvTests
+{
+    [Theory]
+    [InlineData("plain", "plain")]
+    [InlineData("", "")]
+    [InlineData("a,b", "\"a,b\"")]
+    [InlineData("he said \"hi\"", "\"he said \"\"hi\"\"\"")]
+    [InlineData("line1\nline2", "\"line1\nline2\"")]
+    public void Escape_QuotesOnlyWhenNeeded(string input, string expected)
+    {
+        Csv.Escape(input).Should().Be(expected);
+    }
+
+    [Fact]
+    public void Escape_Null_IsEmpty()
+    {
+        Csv.Escape(null).Should().BeEmpty();
+    }
+
+    [Fact]
+    public void Document_JoinsHeaderAndRows_WithCrlf_AndEscapes()
+    {
+        var csv = Csv.Document(
+            new[] { "Name", "Note" },
+            new[]
+            {
+                (IReadOnlyList<string>)new[] { "Kelly", "ok" },
+                new[] { "Flat, fixed", "say \"hi\"" },
+            });
+
+        csv.Should().Be(
+            "Name,Note\r\n" +
+            "Kelly,ok\r\n" +
+            "\"Flat, fixed\",\"say \"\"hi\"\"\"\r\n");
+    }
+}