Fix cash summary quantity bug; add Safe Count as own nav item

- Cash summary: derive total_quantity from total_amount/denomination_value
  so value_entered rows are counted (previously SUM(quantity) returned
  NULL for those rows)
- Safe Count: moved safe_cash out of Float Management tabs into its own
  /safe/* route with a dedicated sidebar entry (Vault icon)
- FloatManagement now only shows Petty Cash and Change Tin tabs

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

frontend/src/components/Layout.tsx

@@ -1,6 +1,6 @@
 import { NavLink, useNavigate } from 'react-router-dom'
 import {
-  Banknote, ClipboardList, BarChart2, Wallet, FileText, Settings, LogOut,
+  Banknote, ClipboardList, BarChart2, Wallet, Vault, FileText, Settings, LogOut,
 } from 'lucide-react'
 import type { User } from '../types'
 
@@ -14,6 +14,7 @@ const navItems = [
   { to: '/history', label: 'History',          icon: ClipboardList },
   { to: '/report',  label: 'Weekly Report',    icon: BarChart2 },
   { to: '/floats',  label: 'Float Management', icon: Wallet },
+  { to: '/safe',    label: 'Safe Count',       icon: Vault },
   { to: '/summary', label: 'Cash Summary',     icon: FileText },
   { to: '/settings',label: 'Settings',         icon: Settings },
 ]