feat: add download button to PDF/image preview header

[tiger0425]

Problem

When users preview a PDF file in the code editor, Chrome's embedded PDF viewer ignores the Content-Disposition header and generates a random UUID filename on download (e.g. a1b2c3d4-e5f6-7890-abcd-ef1234567890.pdf) instead of the actual file name.

This is a known Chrome limitation — the built-in PDF viewer uses an internal blob URL and the HTTP filename parameter has no effect.

Solution

Add an explicit Download button in the preview header bar (headerTopBar). When clicked, <a download="{original-filename}"> forces the browser to save the file with the correct name, bypassing the PDF viewer's internal download logic.

Since the file content is already fetched as a Blob (via URL.createObjectURL) for the iframe preview, the download button reuses the same blob URL — no extra network request needed.

Changes

ui/src/components/code-editor/view/subcomponents/CodeEditorBinaryFile.tsx

Before	After
useBlobUrl called separately inside ImagePreview and PdfPreview	Hook hoisted to parent CodeEditorBinaryFile — single fetch, shared blobUrl
No download button in header	Download icon appears when blobUrl is ready, next to fullscreen/close buttons
Header only had fullscreen + close	New download button using <a download> with original file.name

Testing Notes

• Open any PDF or image file in the code editor preview
• Click the new download icon in the header
• Verify the downloaded file keeps the original filename
• Verify existing preview functionality is unchanged