fix(files): replace instanceof Error checks with toError() and fix skeleton tokens

- Use toError() from @sim/utils/errors across all catch blocks in
  file-viewer.tsx, preview-panel.tsx, and route.ts instead of the
  prohibited `err instanceof Error ? err.message : fallback` pattern
- Fix loading skeleton in files.tsx: bg-white → bg-[var(--surface-2)]
  and shadow-[var(--shadow-medium)] → shadow-medium

apps/sim/app/api/workspaces/[id]/files/[fileId]/content/route.ts

@@ -1,5 +1,6 @@
 import { AuditAction, AuditResourceType, recordAudit } from '@sim/audit'
 import { createLogger } from '@sim/logger'
+import { toError } from '@sim/utils/errors'
 import { type NextRequest, NextResponse } from 'next/server'
 import { getSession } from '@/lib/auth'
 import { withRouteHandler } from '@/lib/core/utils/with-route-handler'
@@ -80,7 +81,7 @@ export const PUT = withRouteHandler(
         file: updatedFile,
       })
     } catch (error) {
-      const errorMessage = error instanceof Error ? error.message : 'Failed to update file content'
+      const errorMessage = toError(error).message || 'Failed to update file content'
       const isNotFound = errorMessage.includes('File not found')
       const isQuotaExceeded = errorMessage.includes('Storage limit exceeded')
       const status = isNotFound ? 404 : isQuotaExceeded ? 402 : 500

apps/sim/app/workspace/[workspaceId]/files/components/file-viewer/file-viewer.tsx

@@ -3,6 +3,7 @@
 import { memo, useCallback, useEffect, useMemo, useReducer, useRef, useState } from 'react'
 import type { OnMount } from '@monaco-editor/react'
 import { createLogger } from '@sim/logger'
+import { toError } from '@sim/utils/errors'
 import { ZoomIn, ZoomOut } from 'lucide-react'
 import dynamic from 'next/dynamic'
 import { Button, Skeleton } from '@/components/emcn'
@@ -1024,7 +1025,7 @@ const IframePreview = memo(function IframePreview({
         setStreamingBuffer(buf)
       } catch (err) {
         if (!cancelled && !(err instanceof DOMException && err.name === 'AbortError')) {
-          const msg = err instanceof Error ? err.message : 'Failed to render PDF'
+          const msg = toError(err).message || 'Failed to render PDF'
           if (streamingBufferRef.current || shouldSuppressStreamingDocumentError(msg)) {
             logger.info('Suppressing transient PDF streaming preview error', { error: msg })
           } else {
@@ -1473,7 +1474,7 @@ const DocxPreview = memo(function DocxPreview({
         }
       } catch (err) {
         if (!cancelled) {
-          const msg = err instanceof Error ? err.message : 'Failed to render document'
+          const msg = toError(err).message || 'Failed to render document'
           logger.error('DOCX render failed', { error: msg })
           setRenderError(msg)
         }
@@ -1545,7 +1546,7 @@ const DocxPreview = memo(function DocxPreview({
             containerRef.current.innerHTML = previousHtml
             setHasRenderedPreview(true)
           }
-          const msg = err instanceof Error ? err.message : 'Failed to render document'
+          const msg = toError(err).message || 'Failed to render document'
           if (previousHtml || shouldSuppressStreamingDocumentError(msg)) {
             logger.info('Suppressing transient DOCX streaming preview error', { error: msg })
           } else {
@@ -1749,7 +1750,7 @@ function PptxPreview({
         )
       } catch (err) {
         if (!cancelled && !(err instanceof DOMException && err.name === 'AbortError')) {
-          const msg = err instanceof Error ? err.message : 'Failed to render presentation'
+          const msg = toError(err).message || 'Failed to render presentation'
           if (shouldSuppressStreamingPptxError(msg)) {
             logger.info('Suppressing transient PPTX streaming preview error', { error: msg })
           } else {
@@ -1801,7 +1802,7 @@ function PptxPreview({
         }
       } catch (err) {
         if (!cancelled) {
-          const msg = err instanceof Error ? err.message : 'Failed to render presentation'
+          const msg = toError(err).message || 'Failed to render presentation'
           logger.error('PPTX render failed', { error: msg })
           setRenderError(msg)
         }
@@ -1911,7 +1912,7 @@ const XlsxPreview = memo(function XlsxPreview({
         }
       } catch (err) {
         if (!cancelled) {
-          const msg = err instanceof Error ? err.message : 'Failed to parse spreadsheet'
+          const msg = toError(err).message || 'Failed to parse spreadsheet'
           logger.error('XLSX parse failed', { error: msg })
           setRenderError(msg)
         }
@@ -1949,7 +1950,7 @@ const XlsxPreview = memo(function XlsxPreview({
         }
       } catch (err) {
         if (!cancelled) {
-          const msg = err instanceof Error ? err.message : 'Failed to parse sheet'
+          const msg = toError(err).message || 'Failed to parse sheet'
           logger.error('XLSX sheet parse failed', { error: msg })
           setRenderError(msg)
         }
@@ -2042,7 +2043,7 @@ const XlsxPreview = memo(function XlsxPreview({
       setIsDirty(false)
       onSaveStatusChangeRef.current?.('saved')
     } catch (err) {
-      logger.error('XLSX save failed', { error: err instanceof Error ? err.message : String(err) })
+      logger.error('XLSX save failed', { error: toError(err).message })
       onSaveStatusChangeRef.current?.('error')
     } finally {
       isSavingRef.current = false

apps/sim/app/workspace/[workspaceId]/files/components/file-viewer/preview-panel.tsx

@@ -8,6 +8,7 @@ import remarkBreaks from 'remark-breaks'
 import remarkGfm from 'remark-gfm'
 import { Streamdown } from 'streamdown'
 import 'streamdown/styles.css'
+import { toError } from '@sim/utils/errors'
 import { generateShortId } from '@sim/utils/id'
 import { Checkbox, CopyCodeButton, highlight, languages } from '@/components/emcn'
 import '@/components/emcn/components/code/code.css'
@@ -270,7 +271,7 @@ const MermaidDiagram = memo(function MermaidDiagram({ definition }: { definition
         }
       } catch (err) {
         if (!cancelled) {
-          setError(err instanceof Error ? err.message : 'Failed to render diagram')
+          setError(toError(err).message || 'Failed to render diagram')
           setSvg(null)
         }
       }

apps/sim/app/workspace/[workspaceId]/files/files.tsx

@@ -1041,7 +1041,7 @@ export function Files() {
           {[0, 1].map((i) => (
             <div
               key={i}
-              className='w-full max-w-[640px] shrink-0 rounded-md bg-white p-8 shadow-[var(--shadow-medium)]'
+              className='w-full max-w-[640px] shrink-0 rounded-md bg-[var(--surface-2)] p-8 shadow-medium'
               style={{ aspectRatio: '1 / 1.414' }}
             >
               <div className='flex flex-col gap-3'>