feat(pdf): default output date format to YYYY-MM-DD

User asked to flip the default from YYYYMMDD to YYYY-MM-DD.
ISO is the better default for an accountant CSV workflow:

- Lexicographic sort = chronological sort (no parsing needed).
- Every spreadsheet tool the user might import into recognises
  it as a real date with no ambiguity (US vs EU readers can't
  disagree on the order).
- Hyphens make the year/month/day boundaries scan-able by eye.

Concrete changes:

- New module constant ``DEFAULT_DATE_FORMAT = "%Y-%m-%d"``,
  used as the default for ``format_date()`` and the
  ``output_date_format`` keyword on
  ``scan_pdf_for_transactions``.
- Page's ``_DATE_FORMAT_CHOICES`` reordered so the ISO entry
  is first (index 0 = default Streamlit selection); YYYYMMDD
  drops to second.
- Custom-strftime input default also flips to ``%Y-%m-%d``.

Tests updated to reflect the new default (``test_dates_formatted_iso_by_default``,
``test_short_dates_get_year_from_period``,
``test_compact_format_round_trip``, plus a new
``test_default_is_iso`` for the format_date helper).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_pdf_extract.py

@@ -272,6 +272,11 @@ class TestFormatDate:
         assert format_date(None) == ""
         assert format_date("") == ""
 
+    def test_default_is_iso(self):
+        # Default format changed to ISO ``YYYY-MM-DD`` — sorts
+        # naturally and parses across every spreadsheet tool.
+        assert format_date("2026-01-13") == "2026-01-13"
+
 
 class TestExtractAccountNumber:
     def test_masked(self):

tests/test_pdf_extract_smoke.py

@@ -136,21 +136,23 @@ class TestScanPdfForTransactions:
             f"{[r.get('raw') for r in rows]}"
         )
 
-    def test_dates_formatted_yyyymmdd_by_default(self, pdf_bytes):
+    def test_dates_formatted_iso_by_default(self, pdf_bytes):
         from src.pdf_extract import scan_pdf_for_transactions
         rows, _ = scan_pdf_for_transactions(pdf_bytes)
-        # Default output format is %Y%m%d
+        # Default output format is %Y-%m-%d — ISO ordering, parses
+        # cleanly in every spreadsheet tool the user might import
+        # this CSV into.
         assert [r["date"] for r in rows] == [
-            "20260115", "20260116", "20260117",
+            "2026-01-15", "2026-01-16", "2026-01-17",
         ]
 
     def test_output_date_format_override(self, pdf_bytes):
         from src.pdf_extract import scan_pdf_for_transactions
         rows, _ = scan_pdf_for_transactions(
-            pdf_bytes, output_date_format="%Y-%m-%d",
+            pdf_bytes, output_date_format="%Y%m%d",
         )
         assert [r["date"] for r in rows] == [
-            "2026-01-15", "2026-01-16", "2026-01-17",
+            "20260115", "20260116", "20260117",
         ]
 
     def test_account_number_field_present_on_every_row(self, pdf_bytes):
@@ -224,12 +226,13 @@ class TestStatementHeaderEndToEnd:
     def test_short_dates_get_year_from_period(self, pdf_bytes):
         from src.pdf_extract import scan_pdf_for_transactions
         rows, _ = scan_pdf_for_transactions(pdf_bytes)
-        # Short ``01/13`` + period ending in 2025 → 20250113.
+        # Short ``01/13`` + period ending in 2025 → 2025-01-13.
         # The period itself isn't surfaced as a column anymore, but
         # the year inference that depends on it still works because
         # extraction happens internally before the per-row stamp.
-        assert rows[0]["date"] == "20250113"
-        assert rows[1]["date"] == "20250116"
+        # Output is in the default ISO format now.
+        assert rows[0]["date"] == "2025-01-13"
+        assert rows[1]["date"] == "2025-01-16"
 
     def test_period_fields_not_in_output(self, pdf_bytes):
         from src.pdf_extract import scan_pdf_for_transactions
@@ -238,12 +241,12 @@ class TestStatementHeaderEndToEnd:
             assert "statement_period_start" not in r
             assert "statement_period_end" not in r
 
-    def test_iso_format_round_trip(self, pdf_bytes):
+    def test_compact_format_round_trip(self, pdf_bytes):
         from src.pdf_extract import scan_pdf_for_transactions
         rows, _ = scan_pdf_for_transactions(
-            pdf_bytes, output_date_format="%Y-%m-%d",
+            pdf_bytes, output_date_format="%Y%m%d",
         )
-        assert rows[0]["date"] == "2025-01-13"
+        assert rows[0]["date"] == "20250113"
 
 
 class TestMultiDateRow: