Include investment holding values in net worth and account balances

- Net worth report, balance sheet, and daily snapshots now add holding
  market values (falling back to cost basis) to investment-type account
  balances (investment, pension, stocks_shares_isa, crypto_wallet)
- Accounts list shows total value for investment accounts with a
  breakdown line ("£X cash + £Y holdings") when both are non-zero
- Add Holding modal gains a "Debit account for this purchase" toggle
  that creates a matching withdrawal transaction, enabling proper cash
  flow tracking for users who fund their brokerage via transfer first
- Both simple (holdings-only) and full cash-flow workflows produce
  correct net worth figures without double-counting

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

frontend/src/pages/investments/AddHoldingModal.tsx

@@ -1,6 +1,7 @@
 import { useState, useEffect } from "react";
 import { X, Search, Loader2 } from "lucide-react";
 import { searchAssets, createHolding, addInvestmentTransaction, AssetSearchResult } from "@/api/investments";
+import { createTransaction } from "@/api/transactions";
 import { useUiStore } from "@/store/uiStore";
 import { format } from "date-fns";
 
@@ -28,6 +29,7 @@ export default function AddHoldingModal({ accounts, onClose, onSuccess }: Props)
   );
 
   const [priceMode, setPriceMode] = useState<"per_share" | "total">("per_share");
+  const [recordCash, setRecordCash] = useState(false);
   const [form, setForm] = useState({
     account_id: investAccounts[0]?.id ?? "",
     quantity: "",
@@ -76,6 +78,18 @@ export default function AddHoldingModal({ accounts, onClose, onSuccess }: Props)
         currency: form.currency,
         date: form.date,
       });
+      if (recordCash) {
+        const totalSpent = qty * price + (parseFloat(form.fees) || 0);
+        await createTransaction({
+          account_id: form.account_id,
+          type: "investment",
+          amount: -totalSpent,
+          currency: form.currency,
+          date: form.date,
+          description: `Buy ${selected.symbol} × ${qty}`,
+          status: "cleared",
+        });
+      }
       onSuccess();
     } catch (e: any) {
       const detail = e?.response?.data?.detail;
@@ -237,6 +251,25 @@ export default function AddHoldingModal({ accounts, onClose, onSuccess }: Props)
             />
           </div>
 
+          {/* Cash purchase toggle */}
+          <label className="flex items-start gap-3 cursor-pointer select-none p-3 rounded-lg border border-border hover:bg-secondary/40 transition-colors">
+            <div className="relative mt-0.5 shrink-0">
+              <input
+                type="checkbox"
+                checked={recordCash}
+                onChange={(e) => setRecordCash(e.target.checked)}
+                className="sr-only"
+              />
+              <div className={`w-9 h-5 rounded-full transition-colors ${recordCash ? "bg-primary" : "bg-secondary border border-input"}`}>
+                <div className={`w-3.5 h-3.5 rounded-full bg-white shadow transition-transform mt-0.5 ${recordCash ? "translate-x-4.5 ml-0.5" : "translate-x-0.5 ml-0.5"}`} />
+              </div>
+            </div>
+            <div>
+              <p className="text-sm font-medium">Debit account for this purchase</p>
+              <p className="text-xs text-muted-foreground mt-0.5">Records a matching withdrawal from the account. Enable this if you transferred cash to this account first and want to track the balance accurately.</p>
+            </div>
+          </label>
+
           {error && <p className="text-destructive text-sm bg-destructive/10 rounded-md px-3 py-2">{error}</p>}
 
           <div className="flex gap-3 pt-1">