feat: add payer control for purchases

- Add explicit payerMemberId field to purchase ledger entries
- Add 'Paid by' selector in mini app purchase add/edit forms
- Default payer to current user when creating new purchases
- Allow admins to change who made existing purchases
- Update backend handlers to accept and persist payerMemberId
- Add i18n translations for 'Paid by' label (EN/RU)

All quality gates pass: build, typecheck, lint, format, test

Co-authored-by: Qwen-Coder <qwen-coder@alibabacloud.com>

packages/ports/src/finance.ts

@@ -191,6 +191,7 @@ export interface FinanceRepository {
     amountMinor: bigint
     currency: CurrencyCode
     description: string | null
+    payerMemberId?: string
     splitMode?: 'equal' | 'custom_amounts'
     participants?: readonly {
       memberId: string