Build plan-shape test data with the typed factories

PlanResults, Plan and usePlanProposal still hand-rolled proposal objects
inline -- roughly 29 nested literals in PlanResults alone. Hand-writing a
deep shape is the pressure that produced 219 `as any` casts across this
suite, so the factories existing but going unused left the trap armed.

Values an assertion depends on stay inline as overrides; the factories
supply only the scaffolding a reader does not need to see. A type gaining
a field now breaks one factory instead of silently passing thirty tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RXMhc9nT6sNz1SL4j8dznL

frontend/src/hooks/usePlanProposal.test.ts

@@ -2,18 +2,11 @@ import { describe, it, expect, beforeEach, vi } from 'vitest'
 import { renderHook, act } from '@testing-library/react'
 import { usePlanProposal } from './usePlanProposal'
 import * as api from '@/services/api'
+import { makePlanProposal } from '../../tests/fixtures'
 
 vi.mock('@/services/api')
 
-const mockProposal = {
-  days: [],
-  outcomes: [],
-  warnings: [],
-  poolBalances: [],
-  vacationBalances: [],
-  surplusAdvice: [],
-  age4Projections: [],
-}
+const mockProposal = makePlanProposal()
 
 describe('usePlanProposal', () => {
   beforeEach(() => {