refactor(skill): remove Office file special handling, merge into unsupported

Remove the Office file placeholder that only showed "Preview will be
supported in a future update" without any download option. Office files
(pdf, doc, docx, xls, xlsx, ppt, pptx) now fall through to the generic
"unsupported file" handler which provides a download button.

Removed:
- OfficeFilePlaceholder component
- isOfficeFile function and OFFICE_EXTENSIONS constant
- isOffice flag from useFileTypeInfo hook
- i18n keys for officePlaceholder

This simplifies the file type handling to just three categories:
- Editable: markdown, code, text files → editor
- Previewable: image, video files → media preview
- Everything else: download button

web/app/components/workflow/skill/editor/office-file-placeholder.tsx

@@ -1,17 +0,0 @@
-import type { FC } from 'react'
-import * as React from 'react'
-import { useTranslation } from 'react-i18next'
-
-const OfficeFilePlaceholder: FC = () => {
-  const { t } = useTranslation('workflow')
-
-  return (
-    <div className="flex h-full w-full items-center justify-center text-text-tertiary">
-      <span className="system-sm-regular">
-        {t('skillEditor.officePlaceholder')}
-      </span>
-    </div>
-  )
-}
-
-export default React.memo(OfficeFilePlaceholder)