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/gui/pages/10_PDF_Extractor.py

@@ -128,8 +128,8 @@ if not _pdf_ok:
 # ---------------------------------------------------------------------------
 
 _DATE_FORMAT_CHOICES = {
-    "YYYYMMDD (20260113)": "%Y%m%d",
     "YYYY-MM-DD (2026-01-13)": "%Y-%m-%d",
+    "YYYYMMDD (20260113)": "%Y%m%d",
     "MM/DD/YYYY (01/13/2026)": "%m/%d/%Y",
     "DD/MM/YYYY (13/01/2026)": "%d/%m/%Y",
     "MMM DD, YYYY (Jan 13, 2026)": "%b %d, %Y",
@@ -173,10 +173,10 @@ with st.expander("Scan options", expanded=False):
     if output_date_format == "__custom__":
         output_date_format = c4.text_input(
             "Custom strftime format",
-            value="%Y%m%d",
+            value="%Y-%m-%d",
             help=(
-                "Python ``strftime`` codes — e.g., ``%Y%m%d`` for "
-                "20260113, ``%Y-%m-%d`` for 2026-01-13."
+                "Python ``strftime`` codes — e.g., ``%Y-%m-%d`` for "
+                "2026-01-13, ``%Y%m%d`` for 20260113."
             ),
         )