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