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>

apps/miniapp/src/miniapp-api.ts

@@ -152,6 +152,7 @@ export interface MiniAppDashboard {
       included: boolean
       shareAmountMajor: string | null
     }[]
+    payerMemberId?: string
   }[]
 }
 
@@ -996,6 +997,7 @@ export async function addMiniAppPurchase(
     description: string
     amountMajor: string
     currency: 'USD' | 'GEL'
+    payerMemberId?: string
     split?: {
       mode: 'equal' | 'custom_amounts'
       participants: readonly {
@@ -1030,6 +1032,7 @@ export async function updateMiniAppPurchase(
     description: string
     amountMajor: string
     currency: 'USD' | 'GEL'
+    payerMemberId?: string
     split?: {
       mode: 'equal' | 'custom_amounts'
       participants: readonly {