Add payoff amount, fix dialog overlay and nav highlight for sub-routes

- Show estimated payoff amount (balance + accrued interest) in loan summary
- Display loan balances as absolute values in red
- Fix dialog overlay blocking UI after edit by closing in finally with nextTick
- Highlight parent nav item when viewing sub-routes (e.g. /loans/:id)

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

frontend/src/components/layout/AppLayout.vue

@@ -159,6 +159,8 @@ const navItems = [
 
 function isNavActive(navRoute: string) {
   if (route.path === navRoute) return true
+  // Highlight parent nav for sub-routes (e.g. /loans/:id highlights Loans)
+  if (navRoute !== '/' && route.path.startsWith(navRoute + '/')) return true
   // Highlight "Transactions" when viewing account-scoped transactions
   if (navRoute === '/transactions' && route.name === 'account-transactions') return true
   return false