Add investment holdings CRUD (create, edit, delete)

Add POST/PUT/DELETE endpoints to InvestmentsController with find-or-create
Security by normalized symbol, price upsert helper, and ownership checks.
Frontend gets add/edit dialog with per-share cost basis input, delete
confirmation, actions column on holdings table, and success/error snackbar.

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

frontend/src/services/investments.ts

@@ -1,5 +1,5 @@
 import api from './api'
-import type { InvestmentHolding, InvestmentPerformance } from '@/types'
+import type { InvestmentHolding, InvestmentPerformance, CreateInvestmentHoldingRequest, UpdateInvestmentHoldingRequest } from '@/types'
 
 export const investmentsApi = {
   getHoldings: (accountId: string) =>
@@ -7,4 +7,13 @@ export const investmentsApi = {
 
   getPerformance: (accountId: string) =>
     api.get<InvestmentPerformance>(`/investments/${accountId}/performance`),
+
+  createHolding: (accountId: string, data: CreateInvestmentHoldingRequest) =>
+    api.post<InvestmentHolding>(`/investments/${accountId}/holdings`, data),
+
+  updateHolding: (accountId: string, holdingId: string, data: UpdateInvestmentHoldingRequest) =>
+    api.put<InvestmentHolding>(`/investments/${accountId}/holdings/${holdingId}`, data),
+
+  deleteHolding: (accountId: string, holdingId: string) =>
+    api.delete(`/investments/${accountId}/holdings/${holdingId}`),
 }