feat: group report transactions by report granularity

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.

src/common/types.ts

@@ -11,14 +11,6 @@ export interface ExpenseItem {
   updated_at?: string;
 }
 
-export interface GroupedMonth {
-  key: string;
-  items: ExpenseItem[];
-  spent: number;
-  income: number;
-  currency: string;
-}
-
 export interface TxnFieldConfigs {
   entity: FieldConfig;
   amount: FieldConfig;