Improve loading speed of desktop app (#4865)

# Description of Changes
Improve loading speed of desktop app by loading a default config until
the backend has spawned.

frontend/src/desktop/hooks/useBackendHealth.ts

@@ -1,91 +1,24 @@
-import { useState, useEffect, useCallback } from 'react';
-import { tauriBackendService } from '@app/services/tauriBackendService';
-
-export type BackendStatus = 'starting' | 'healthy' | 'unhealthy' | 'stopped';
-
-interface BackendHealthState {
-  status: BackendStatus;
-  message?: string;
-  lastChecked?: number;
-  isChecking: boolean;
-  error: string | null;
-}
+import { useEffect, useState, useCallback } from 'react';
+import { backendHealthMonitor } from '@app/services/backendHealthMonitor';
+import type { BackendHealthState } from '@app/types/backendHealth';
 
 /**
- * Hook to monitor backend health status with retries
+ * Hook to read the shared backend health monitor state.
+ * All consumers subscribe to a single poller managed by backendHealthMonitor.
  */
-export function useBackendHealth(pollingInterval = 5000) {
-  const [health, setHealth] = useState<BackendHealthState>({
-    status: tauriBackendService.isBackendRunning() ? 'healthy' : 'stopped',
-    isChecking: false,
-    error: null,
-  });
-
-  const checkHealth = useCallback(async () => {
-    setHealth((current) => ({
-      ...current,
-      status: current.status === 'healthy' ? 'healthy' : 'starting',
-      isChecking: true,
-      error: 'Backend starting up...',
-      lastChecked: Date.now(),
-    }));
-
-    try {
-      const isHealthy = await tauriBackendService.checkBackendHealth();
-
-      setHealth({
-        status: isHealthy ? 'healthy' : 'unhealthy',
-        lastChecked: Date.now(),
-        message: isHealthy ? 'Backend is healthy' : 'Backend is unavailable',
-        isChecking: false,
-        error: isHealthy ? null : 'Backend offline',
-      });
-
-      return isHealthy;
-    } catch (error) {
-      console.error('[BackendHealth] Health check failed:', error);
-      setHealth({
-        status: 'unhealthy',
-        lastChecked: Date.now(),
-        message: 'Backend is unavailable',
-        isChecking: false,
-        error: 'Backend offline',
-      });
-      return false;
-    }
-  }, []);
+export function useBackendHealth() {
+  const [health, setHealth] = useState<BackendHealthState>(() => backendHealthMonitor.getSnapshot());
 
   useEffect(() => {
-    let isMounted = true;
+    return backendHealthMonitor.subscribe(setHealth);
+  }, []);
 
-    const initialize = async () => {
-      setHealth((current) => ({
-        ...current,
-        status: tauriBackendService.isBackendRunning() ? 'starting' : 'stopped',
-        isChecking: true,
-        error: 'Backend starting up...',
-      }));
-
-      await checkHealth();
-      if (!isMounted) return;
-    };
-
-    initialize();
-
-    const interval = setInterval(() => {
-      if (!isMounted) return;
-      void checkHealth();
-    }, pollingInterval);
-
-    return () => {
-      isMounted = false;
-      clearInterval(interval);
-    };
-  }, [checkHealth, pollingInterval]);
+  const checkHealth = useCallback(async () => {
+    return backendHealthMonitor.checkNow();
+  }, []);
 
   return {
     ...health,
-    isHealthy: health.status === 'healthy',
     checkHealth,
   };
 }