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/Sidebar.tsx

@@ -77,11 +77,20 @@ const navSections = [
   },
 ];
 
-export function Sidebar() {
+interface SidebarProps {
+  onNavigate?: () => void;
+}
+
+export function Sidebar({ onNavigate }: SidebarProps) {
   const navigate = useNavigate();
   const location = useLocation();
   const user = useAuthStore((s) => s.user);
 
+  const go = (path: string) => {
+    navigate(path);
+    onNavigate?.();
+  };
+
   return (
     <ScrollArea p="sm">
       {navSections.map((section, sIdx) => (
@@ -109,7 +118,7 @@ export function Sidebar() {
                     key={child.path}
                     label={child.label}
                     active={location.pathname === child.path}
-                    onClick={() => navigate(child.path)}
+                    onClick={() => go(child.path)}
                   />
                 ))}
               </NavLink>
@@ -119,7 +128,7 @@ export function Sidebar() {
                 label={item.label}
                 leftSection={<item.icon size={18} />}
                 active={location.pathname === item.path}
-                onClick={() => navigate(item.path!)}
+                onClick={() => go(item.path!)}
               />
             ),
           )}
@@ -136,7 +145,7 @@ export function Sidebar() {
             label="Admin Panel"
             leftSection={<IconCrown size={18} />}
             active={location.pathname === '/admin'}
-            onClick={() => navigate('/admin')}
+            onClick={() => go('/admin')}
             color="red"
           />
         </>