Compute Sum/Count/Avg/Min/Max/First/Last per accordion group in the
report view (mirroring backend ReportMetrics.compute_metrics signed
semantics) via new computeTxnMetrics helper and a showMetrics prop on
TransactionList. Remove the now-redundant top-level metric strip from
ReportViewer and restore its Outflows/Inflows/Transactions stat cards.
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.
