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>

src/pdf_extract.py

@@ -520,6 +520,13 @@ def _find_amount_tokens(
     return out
 
 
+DEFAULT_DATE_FORMAT = "%Y-%m-%d"
+"""ISO-8601-style ``YYYY-MM-DD``. Default for output date columns
+because it sorts lexicographically, parses in every spreadsheet
+tool the user might import the CSV into, and is unambiguous
+across US/EU readers."""
+
+
 def format_amount(value, places: int = 2) -> str:
     """Render an amount value as a fixed-precision string.
 
@@ -549,7 +556,7 @@ def format_amount(value, places: int = 2) -> str:
     return str(value)
 
 
-def format_date(iso_str: str | None, fmt: str = "%Y%m%d") -> str:
+def format_date(iso_str: str | None, fmt: str = DEFAULT_DATE_FORMAT) -> str:
     """Convert an ISO ``YYYY-MM-DD`` date string to *fmt*.
 
     Returns the input unchanged if it's not parseable as ISO,
@@ -840,7 +847,7 @@ def scan_pdf_for_transactions(
     date_formats: list[str] | None = None,
     y_tolerance: float = 3.0,
     merge_multiline_descriptions: bool = True,
-    output_date_format: str = "%Y%m%d",
+    output_date_format: str = DEFAULT_DATE_FORMAT,
     filename_year_hint: int | None = None,
     year_override: int | None = None,
 ) -> tuple[list[dict[str, Any]], list[str]]:
@@ -850,7 +857,7 @@ def scan_pdf_for_transactions(
     amount pattern. Each returned record looks like::
 
         {
-          "date": "20260115",     # output_date_format applied
+          "date": "2026-01-15",   # output_date_format applied
           "description": "...",
           "amount_1": 4.50,
           "amount_2": 1000.00,    # if a second amount was found