feat(artifacts): open binary files & the folder with the OS app

[juacker]

What

Two artifacts-drawer improvements (PR A of the three you raised):

1. Open binary/rich files with the OS app (fixes the PDF error)

Clicking a .pdf (or any non-text file) in the artifacts tree used to open the in-app text preview, which then failed with "does not contain UTF-8 text". Now such files launch the OS default application instead.

• viewer_for_path gains an "external" class for documents (pdf, doc/docx, xls/xlsx, ppt/pptx, odt/ods/odp, rtf, epub), images (png/jpg/gif/webp/…), audio/video, archives (zip/tar/7z/…), and binaries (exe/dll/so/wasm/jar/sqlite/db/parquet/fonts).
• Clicking an external tree row calls open_workspace_path(ws, path, "system") — the injection-safe OS-open machinery from fix(system-apps): make "open in editor/terminal" cross-platform #80 (xdg-open / explorer.exe / open).
• Belt-and-braces: the preview error state now shows an "Open with the system app" button, so an unrecognized binary extension that still falls through to the text reader has an escape hatch.
• Note: images are routed externally for now (no inline image preview yet — that's a separate follow-up).

2. Open the workspace folder in the file browser

New icon in the artifacts drawer header (next to open-in-editor / open-in-terminal) that opens the workspace root in the OS file manager — open_workspace_path(ws, null, "system"), which on a directory opens Files/Explorer/Finder.

Verification

• Backend: cargo fmt + new unit test viewer_for_path_classifies_binary_as_external (pdf/png/zip/sqlite → external; md/rs/json unchanged).
• FE: tsc / eslint / vitest (132) / vite build clean.
• No bindings change (viewer was already a string field).

Test locally

• Click a PDF/image/zip artifact → it opens in the OS app, no error.
• Artifacts drawer header → the new folder icon opens your file manager at the workspace root.
• (Edge) open a binary with an extension not in the list via the preview → the error card offers "Open with the system app".

Next up (separately, as agreed): PR B — drag-and-drop copy of artifacts between workspaces.