fix(pdf): consistent 2-decimal amount precision in display and CSV

User reported amounts losing trailing zeros — 4.50 rendering as
4.5, 1000.00 as 1000 — on the same statement. Classic float
display issue: Python's native ``repr(4.5)`` drops the
``.0``, and pandas / Streamlit happily show that
inconsistency cell-by-cell.

Two layers of fix, internal type stays ``float`` for arithmetic:

**Display.** ``st.column_config.NumberColumn(format="%.2f")``
applied programmatically to every ``amount_*`` column on the
data_editor. Every numeric amount now shows with exactly two
decimal places regardless of trailing zeros.

**CSV export.** Pandas' default float-to-CSV writer also drops
trailing zeros (the same issue an accountant would see when
opening the file in Excel). Before serialising, each amount
column is mapped through the new ``format_amount`` helper —
returns ``f"{v:.2f}"`` for numerics, empty string for
None/NaN/inf, ``str(value)`` for booleans (guards the
``True → "1.00"`` foot-gun since ``bool`` is an ``int``
subclass), and passes through any string the scanner kept
because parsing failed (e.g. ``(4.50)`` when parens-negative is
off — user can correct in the editor before re-exporting).

``format_amount`` lives in ``src/pdf_extract.py`` so it's
testable in isolation (the page module can't easily be unit
tested because of its Streamlit import chain). 8 new tests
cover the trailing-zeros case, negatives, None/empty,
string-passthrough, bool guard, NaN/inf, and the ``places``
parameter.

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

tests/test_pdf_extract.py

@@ -22,6 +22,7 @@ from src.pdf_extract import (
     _infer_year_for_short_date,
     cluster_rows,
     extract_statement_metadata,
+    format_amount,
     format_date,
     parse_amount,
     parse_date,
@@ -214,6 +215,43 @@ class TestFindAmountTokens:
 # the building blocks; smoke tests pin the wiring.
 
 
+class TestFormatAmount:
+    """Two-decimal-place consistency at the display + export layer."""
+
+    def test_drops_no_trailing_zeros(self):
+        # The bug: 4.5 should NOT render as "4.5" — accountants
+        # need consistent precision across rows.
+        assert format_amount(4.5) == "4.50"
+        assert format_amount(12.0) == "12.00"
+        assert format_amount(1000) == "1000.00"
+
+    def test_negatives(self):
+        assert format_amount(-40.0) == "-40.00"
+        assert format_amount(-4.5) == "-4.50"
+
+    def test_none_and_empty(self):
+        assert format_amount(None) == ""
+        assert format_amount("") == ""
+
+    def test_string_passthrough(self):
+        # ``(4.50)`` was preserved by the scanner because parsing
+        # failed; the user sees the raw text and can fix in editor.
+        assert format_amount("(4.50)") == "(4.50)"
+
+    def test_bool_doesnt_render_as_number(self):
+        # bool is an int subclass — guard prevents True → "1.00".
+        assert format_amount(True) == "True"
+        assert format_amount(False) == "False"
+
+    def test_nan_inf_become_empty(self):
+        assert format_amount(float("nan")) == ""
+        assert format_amount(float("inf")) == ""
+
+    def test_custom_places(self):
+        assert format_amount(4.5, places=4) == "4.5000"
+        assert format_amount(4.567, places=0) == "5"
+
+
 class TestFormatDate:
     def test_yyyymmdd(self):
         assert format_date("2026-01-13", "%Y%m%d") == "20260113"