Add optional Google Sheets export for the masterplan

Lets organizers automatically populate a shared Google Sheet (the same kind
they previously built by hand) with an Overview tab and one tab per group,
instead of printing dicts. Group tabs are named after all their members
(not a single host) and show each group's route, course times, and guests
with allergies. The Overview tab also gets configurable Meal Times, Support
Contacts, and Info sections, passed through as plain data from
tatami_masterplan.py. Export is fully opt-in via GOOGLE_SHEETS_CREDENTIALS_FILE
and GOOGLE_SHEETS_SPREADSHEET_ID; without them, behavior is unchanged.

pyproject.toml

@@ -5,6 +5,7 @@ description = "Tool for Arranging Tasty Appointments, Meetings & Invitations"
 readme = "README.md"
 requires-python = ">=3.13"
 dependencies = [
+    "gspread>=6.2.1",
     "numpy>=2.2.4",
     "pandas>=2.2.3",
     "pandas-stubs>=2.2.3.250308",