Quality-of-life enhancements: CSV import/export, opening balances, interest rates, mobile UX

- CSV import/export for Units, Projects, and Vendors with match-on-name/number upsert
- Cash Flow report toggle for Cash Only vs Cash + Investments
- Per-account and bulk opening balance setting with as-of date
- Interest rate field on normal accounts with estimated monthly/annual interest display
- Mobile sidebar auto-close on navigation
- Shared CSV parsing/export utility extracted to frontend/src/utils/csv.ts

DB migration needed for existing tenants:
  ALTER TABLE accounts ADD COLUMN IF NOT EXISTS interest_rate DECIMAL(6,4);

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

frontend/src/components/layout/AppLayout.tsx

@@ -14,7 +14,7 @@ import { Sidebar } from './Sidebar';
 import logoSrc from '../../assets/logo.svg';
 
 export function AppLayout() {
-  const [opened, { toggle }] = useDisclosure();
+  const [opened, { toggle, close }] = useDisclosure();
   const { user, currentOrg, logout } = useAuthStore();
   const navigate = useNavigate();
 
@@ -98,7 +98,7 @@ export function AppLayout() {
       </AppShell.Header>
 
       <AppShell.Navbar>
-        <Sidebar />
+        <Sidebar onNavigate={close} />
       </AppShell.Navbar>
 
       <AppShell.Main>