Fix backend issues in desktop app (#4995)

# Description of Changes
Fixes two distinct but related issues in the backend of the desktop app:
- Correctly shows tools as unavaialable when the backend doesn't have
the dependencies or has disabled them etc. (same as web version - this
primarily didn't work on desktop because the app spawns before the
backend is running)
- Fixes infinite re-rendering issues caused by the app polling whether
the backend is healthy or not

frontend/src/core/hooks/useBackendHealth.ts

@@ -4,8 +4,6 @@ export function useBackendHealth(): BackendHealthState {
   return {
     status: 'healthy',
     message: null,
-    isChecking: false,
-    lastChecked: Date.now(),
     error: null,
     isHealthy: true,
   };