Brand the export header band and freeze the header row 🟩

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

infrastructure/xlsx/branding.py

@@ -11,6 +11,8 @@ palette once design supplies it.
 
 from __future__ import annotations
 
+from openpyxl.styles import Alignment, Font, PatternFill
+from openpyxl.utils import get_column_letter
 from openpyxl.worksheet.worksheet import Worksheet
 
 # openpyxl colours are 8-char ARGB. Placeholder Monday-style palette.
@@ -24,4 +26,26 @@ _COLUMN_WIDTH: int = 18
 def style_sheet(worksheet: Worksheet) -> None:
     """Paint the Monday-style branding onto a populated worksheet: header band +
     font, frozen header row, banded rows, and column widths."""
-    raise NotImplementedError
+    header_fill = PatternFill(
+        start_color=HEADER_FILL, end_color=HEADER_FILL, fill_type="solid"
+    )
+    header_font = Font(bold=True, color=HEADER_FONT)
+    for cell in worksheet[1]:
+        cell.fill = header_fill
+        cell.font = header_font
+        cell.alignment = Alignment(horizontal="left", vertical="center")
+
+    # Keep the header visible while scrolling a long portfolio.
+    worksheet.freeze_panes = "A2"
+
+    band_fill = PatternFill(
+        start_color=ROW_BAND_FILL, end_color=ROW_BAND_FILL, fill_type="solid"
+    )
+    for row_index in range(3, worksheet.max_row + 1, 2):
+        for cell in worksheet[row_index]:
+            cell.fill = band_fill
+
+    for column_index in range(1, worksheet.max_column + 1):
+        worksheet.column_dimensions[get_column_letter(column_index)].width = (
+            _COLUMN_WIDTH
+        )

infrastructure/xlsx/scenario_export_workbook.py

@@ -15,6 +15,7 @@ from typing import Sequence
 from openpyxl import Workbook
 
 from domain.scenario_export.scenario_sheet import ExportSheet
+from infrastructure.xlsx.branding import style_sheet
 
 # Excel forbids these in a sheet title and caps titles at 31 characters.
 _ILLEGAL_SHEET_CHARS = re.compile(r"[:\\/?*\[\]]")
@@ -50,6 +51,7 @@ def render_workbook(sheets: Sequence[tuple[str, ExportSheet]]) -> bytes:
         worksheet.append(list(sheet.columns))
         for row in sheet.rows:
             worksheet.append([row.get(column, "") for column in sheet.columns])
+        style_sheet(worksheet)
 
     buffer = BytesIO()
     workbook.save(buffer)