Enable ESLint no-unused-vars rule (#4367)

# Description of Changes
Enable ESLint [no-unused-vars
rule](https://typescript-eslint.io/rules/no-unused-vars/)

frontend/src/services/pdfWorkerManager.ts

@@ -93,7 +93,7 @@ class PDFWorkerManager {
       if (loadingTask) {
         try {
           loadingTask.destroy();
-        } catch (destroyError) {
+        } catch {
           // Ignore errors
         }
       }
@@ -110,7 +110,7 @@ class PDFWorkerManager {
         pdf.destroy();
         this.activeDocuments.delete(pdf);
         this.workerCount = Math.max(0, this.workerCount - 1);
-      } catch (error) {
+      } catch {
         // Still remove from tracking even if destroy failed
         this.activeDocuments.delete(pdf);
         this.workerCount = Math.max(0, this.workerCount - 1);
@@ -166,7 +166,7 @@ class PDFWorkerManager {
     this.activeDocuments.forEach(pdf => {
       try {
         pdf.destroy();
-      } catch (error) {
+      } catch {
         // Ignore errors
       }
     });