Implement Phase 3: financial features frontend (budgets, scheduled transactions, loans)

Add API service files and replace stub views with full implementations:
- Budgets: month navigator, budget items table with progress bars, create/edit/delete
- Scheduled Transactions: data table with skip/post-now actions, add/edit dialog
- Loans: loan list, detail view with amortization schedule, payoff scenario calculator

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/services/budgets.ts

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+import api from './api'
+import type { Budget, BudgetItem } from '@/types'
+
+export const budgetsApi = {
+  getByMonth: (year: number, month: number) =>
+    api.get<Budget>(`/budgets/${year}/${month}`),
+
+  create: (data: { year: number; month: number; notes?: string; items: { categoryId: string; budgetedAmount: number }[] }) =>
+    api.post<Budget>('/budgets', data),
+
+  update: (id: string, data: { notes?: string; items: { categoryId: string; budgetedAmount: number }[] }) =>
+    api.put<Budget>(`/budgets/${id}`, data),
+
+  delete: (id: string) =>
+    api.delete(`/budgets/${id}`),
+
+  getActuals: (id: string) =>
+    api.get<BudgetItem[]>(`/budgets/${id}/actual`),
+}

frontend/src/services/loans.ts

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+import api from './api'
+import type { LoanDetail, PayoffScenario } from '@/types'
+
+export const loansApi = {
+  getLoanDetail: (accountId: string) =>
+    api.get<LoanDetail>(`/loans/${accountId}/amortization`),
+
+  createLoanDetail: (accountId: string, data: any) =>
+    api.post<LoanDetail>(`/loans/${accountId}`, data),
+
+  updateLoanDetail: (accountId: string, data: any) =>
+    api.put<LoanDetail>(`/loans/${accountId}`, data),
+
+  calculatePayoff: (accountId: string, data: { extraMonthlyPayment?: number; lumpSumPayment?: number; lumpSumDate?: string }) =>
+    api.post<PayoffScenario>(`/loans/${accountId}/payoff-scenario`, data),
+}

frontend/src/services/scheduled.ts

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+import api from './api'
+import type { ScheduledTransaction } from '@/types'
+
+export const scheduledApi = {
+  getAll: () =>
+    api.get<ScheduledTransaction[]>('/scheduled-transactions'),
+
+  getById: (id: string) =>
+    api.get<ScheduledTransaction>(`/scheduled-transactions/${id}`),
+
+  create: (data: any) =>
+    api.post<ScheduledTransaction>('/scheduled-transactions', data),
+
+  update: (id: string, data: any) =>
+    api.put<ScheduledTransaction>(`/scheduled-transactions/${id}`, data),
+
+  delete: (id: string) =>
+    api.delete(`/scheduled-transactions/${id}`),
+
+  skip: (id: string) =>
+    api.post<ScheduledTransaction>(`/scheduled-transactions/${id}/skip`),
+
+  postNow: (id: string) =>
+    api.post(`/scheduled-transactions/${id}/post-now`),
+}