Add mobile/desktop toggle to the search engine preview modal

### Description

The search engine live preview shipped in #137 (`components/open-graph/PreviewModal.tsx` → `SearchPreview.tsx`) renders a single, generic result-snippet layout. Issue #161's MVP requirements call for the preview to support both mobile and desktop views, since search result snippet truncation and layout differ between the two.

### Use Case

Editors want to see how a page will actually appear in search results on both mobile and desktop before publishing, since title/description truncation lengths differ meaningfully between the two surfaces.

### Proposed Solution

Add a toggle (e.g. tabs similar to the existing Search/Social tabs) to `SearchPreview.tsx` that switches between a mobile-style and desktop-style rendering of the snippet, using the appropriate truncation lengths/layout for each.

### Related

- #161 (parent MVP tracking issue)
- #137 (original search/social preview implementation)

Start with components/open-graph/PreviewModal.tsx and the SearchPreview.tsx component it renders. Compare the existing Search/Social tab behavior mentioned in the issue, then add a mobile/desktop switch in SearchPreview.tsx. Done means the search preview can show distinct mobile-style and desktop-style snippet layouts with appropriate truncation differences.

索引モデルが issue の本文から書いたものです。