Client UX: toast system, confirm dialogs, load-error banners, nav fix, 404 page

- New ToastProvider/useToast (self-built, no dep): success/error toasts replace
  all five alert() calls, with server error messages surfaced
- New ConfirmProvider/useConfirm promise-based dialog replaces all six
  window.confirm sites, showing what is being deleted; Escape/overlay cancels
- Dashboard + BreedingCalendar loaders use Promise.allSettled with a
  danger banner + Retry instead of silently blanking on fetch failure
- Settings load failure now surfaces a toast instead of vanishing
- Fix BreedingCalendar females list: read paginated {data} shape (was .dogs,
  so 'Start Heat Cycle' never listed any females)
- Nav links highlight on detail routes (/dogs/123, /pedigree/5)
- Catch-all 404 route with link back to Dashboard

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

client/src/hooks/useSettings.jsx

@@ -9,13 +9,18 @@ export function SettingsProvider({ children }) {
     kennel_tagline: '',
   })
   const [loading, setLoading] = useState(true)
+  // Surfaced by consumers inside the ToastProvider (this provider sits above it)
+  const [loadError, setLoadError] = useState(false)
 
   useEffect(() => {
     axios.get('/api/settings')
       .then(res => {
         setSettings(prev => ({ ...prev, ...res.data }))
       })
-      .catch(() => {})
+      .catch((err) => {
+        console.error('Failed to load settings:', err)
+        setLoadError(true)
+      })
       .finally(() => setLoading(false))
   }, [])
 
@@ -25,7 +30,7 @@ export function SettingsProvider({ children }) {
   }
 
   return (
-    <SettingsContext.Provider value={{ settings, saveSettings, loading }}>
+    <SettingsContext.Provider value={{ settings, saveSettings, loading, loadError }}>
       {children}
     </SettingsContext.Provider>
   )