fix(pdf): drop statement_period_start/end columns from output

User asked to remove them — the two columns repeated the same
value on every row from a given statement, took up screen space
in the editor, and offered limited value once the date column
already carries the inferred full date.

What's kept:
- ``account_number`` — still stamped onto every row so multi-
  statement CSVs are self-attributing
- ``extract_statement_metadata`` — still runs every scan because
  ``period_end`` is the source of the year inference that binds
  Chase-style short ``01/13`` dates to ``20250113``
- ``_extract_statement_period`` and its tests — period
  detection itself isn't going anywhere, just its appearance in
  the output rows

What's removed:
- ``record["statement_period_start"]`` / ``record["statement_period_end"]``
  assignments in ``scan_pdf_for_transactions``
- The two columns from the page's column-ordering setup
- Tests pinning their presence; replaced with assertions that
  they're explicitly absent

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

src/gui/pages/10_PDF_Extractor.py

@@ -427,7 +427,7 @@ else:
 
     # Order columns so the user-facing fields are leftmost; raw +
     # internals are last and easy to scroll past or unselect at
-    # download time. Statement metadata sits with the transaction
+    # download time. ``account_number`` sits with the transaction
     # detail since it's per-row context an accountant typically
     # wants alongside the amounts.
     front = [
@@ -435,11 +435,7 @@ else:
         "description",
     ]
     amount_cols = sorted(c for c in df.columns if c.startswith("amount_"))
-    metadata_cols = [
-        "account_number",
-        "statement_period_start",
-        "statement_period_end",
-    ]
+    metadata_cols = ["account_number"]
     tail = ["source_file", "page", "raw"]
     ordered = [
         c for c in front + amount_cols + metadata_cols + tail

src/pdf_extract.py

@@ -782,15 +782,15 @@ def scan_pdf_for_transactions(
           "page": 1,
           "raw": "01/15/2026 Coffee $4.50",
           "account_number": "****1234",      # from header
-          "statement_period_start": "20260101",
-          "statement_period_end": "20260131",
         }
 
-    Header metadata (``account_number`` /
-    ``statement_period_start`` / ``statement_period_end``) is
-    extracted once per PDF and stamped onto every detected row.
-    That way a multi-statement CSV remains attributable per row
-    when it's reshaped or imported elsewhere.
+    Account number is extracted from the statement header once
+    per PDF and stamped onto every detected row so the CSV is
+    self-attributing when statements are combined. The statement
+    period IS detected (used internally for year inference on
+    short dates like "01/13") but isn't surfaced as a per-row
+    column — the inferred year already lives in the ``date``
+    field.
 
     Short dates without a year (``01/13``, ``Jan 13``) are bound
     to the year of the statement period's end before formatting.
@@ -915,15 +915,14 @@ def scan_pdf_for_transactions(
             if not _has_real_transaction_amount(record):
                 continue
 
-            # Stamp the header metadata onto every kept row so the
-            # CSV is self-attributing.
+            # Stamp the account number onto every kept row so the
+            # CSV is self-attributing when statements are combined.
+            # The period start/end aren't surfaced per row — they're
+            # used only for the year-inference fallback above
+            # (binding short dates like "01/13" to the statement's
+            # year) but downstream the date column already carries
+            # the inferred full date.
             record["account_number"] = metadata["account_number"] or ""
-            record["statement_period_start"] = format_date(
-                metadata["period_start"], output_date_format,
-            )
-            record["statement_period_end"] = format_date(
-                metadata["period_end"], output_date_format,
-            )
 
             out_rows.append(record)
             prev = record