[FEATURE]: web: file preview has no download action, binary files cannot be saved #60180

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Describe the enhancement you want to request

Problem

The web app (opencode pair → browser) file preview is view-only: there is no download action in the preview header, the file tree, or any tab menu. Files the agent generated on the server cannot be saved from the browser.

• Text files: select-all + copy is the only way out.
• Binary files (zip, pdf, docx, images, fonts, …): the preview shows the binary placeholder and there is no route to the bytes at all.
• The only built-in download I could find is view.download in the legacy artifact toolbar (packages/gui-plugins/src/file/artifact-toolbar.tsx), and it only renders while props.text !== undefined. The V2 file preview (packages/session-ui/src/v2/components/session-review-file-preview-v2.tsx) exposes no toolbar actions, so in current V2 there is no download entry point at all.
• API side: the server.fs group only has read, list, find (plus experimental write / watch). 2.1.2's /openapi.json and current dev (packages/server/src/handlers/fs.ts) both have no download route.

This matters most for remote / paired sessions: the person in the browser often has no shell on the server, so "just scp it" is not available.

Proposed solution

Add a download action (button and/or context-menu item) to the file preview and file tree that streams the raw bytes from fs.read:

1. Server: add GET /api/fs/download/* (or a download flag on fs.read) returning Content-Disposition: attachment; filename="…", with RFC 5987 encoding for non-ASCII names. serveFile already streams with Range support, so large files should not be buffered in the app.
2. App: a Download item in the file preview header and in the file tree row menu that navigates to that URL. Using a navigation, not fetch + blob, keeps memory flat for large generated artifacts.
3. Keep it available when the preview falls back to the binary placeholder — that is the case that is impossible today.

Prior art

• [FEATURE]: file upload, download and delete in the file manager #39878 (upload/download/delete in the file manager) was closed automatically after 60 days of inactivity.
• feat(app): add file upload, download and delete to file manager #39879 ("feat(app): add file upload, download and delete to file manager") implemented this, including fs.download with sanitized filenames and a 2 GB upload limit, but was closed unmerged.
• [FEATURE]: Add ability to download generated file from remote server in Desktop preview #58957 (desktop, remote preview download) was closed as a duplicate; app: the attachment card in a sent message is display-only — the uploaded file cannot be opened or downloaded #59896 covers message attachments, a different surface.

I am happy to rebase and re-submit #39879 against current dev, or to open a smaller PR that adds only the download endpoint plus the preview header item, whichever is preferred.

Workaround used today

A userscript that re-fetches /api/fs/read/<path>?location[directory]=<dir> and saves the blob through a[download]. It works, but no user should need one for a basic "save this generated file" action.

Environment

• opencode 2.1.2, Linux x64; the same gap exists in current dev.
• Web app accessed over opencode pair in a desktop browser.