fix: resolve eslint v10 errors in app (no-useless-assignment, preserve-caught-error)

Author: Metalhearf

apps/fluux/src/components/LinkPreviewCard.tsx

@@ -15,7 +15,7 @@ export function LinkPreviewCard({ preview, onLoad }: LinkPreviewCardProps) {
   const [imageError, setImageError] = useState(false)
 
   // Extract domain from URL for display
-  let domain = ''
+  let domain: string
   try {
     const url = new URL(preview.url)
     domain = url.hostname.replace(/^www\./, '')

apps/fluux/src/components/conversation/MessageList.test-utils.ts

@@ -354,7 +354,7 @@ export class ScenarioBuilder {
       try {
         await step.fn()
       } catch (error) {
-        throw new Error(`Scenario failed at "${step.label}": ${error}`)
+        throw new Error(`Scenario failed at "${step.label}": ${error}`, { cause: error })
       }
     }
   }