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/adapters-db/src/finance-repository.ts

@@ -441,6 +441,11 @@ export function createDbFinanceRepository(
                   participantSplitMode: input.splitMode
                 }
               : {}),
+            ...(input.payerMemberId
+              ? {
+                  senderMemberId: input.payerMemberId
+                }
+              : {}),
             needsReview: 0,
             processingStatus: 'confirmed',
             parserError: null