Make the outer accordion grouping in TransactionList granularity-aware
instead of hardcoded months. TransactionList accepts a granularity prop
(weekly/monthly/quarterly/yearly, default monthly); ReportViewer passes
the report's granularity so a weekly report buckets transactions by ISO
week, quarterly by quarter, yearly by year. Expense page keeps monthly
grouping. Add periodKey/periodLabel/groupByPeriod/toPeriodGranularity to
src/common/utils/transactions.ts and drop the dead groupByMonth/
GroupedMonth helpers.
Collapse the Expenses page into a single src/Expense.tsx file and rename
the Reports page to src/Reports/Report.tsx, extracting their shared
transaction-list UI, date/grouping helpers, field configs, and types
into an expense/report-agnostic src/common module. Update ReportViewer
and GenerateReportPanel to consume the shared components, rewire
main.jsx imports, and remove the old src/Expense/ folder and
src/Reports/Reports.tsx.
