Support exporting project files/folders as a zip archive

## Self Checks

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## 1. Is this request related to a problem you're experiencing?

There is currently no way to export project files out of Syncless. Users who work with the agent on `cloud:/work` or a project's file area have no built-in way to download individual files, a folder, or the whole project as an archive — they have to manually copy files out one by one (or ask the agent to do it), which is slow and error-prone for anything beyond a single file.

## 2. What is your proposed solution?

Add an **export** action to the existing project files view:

- In the current "project files" location, allow the user to select one or more files, or a folder.
- Provide an **Export** action that packages the selection into a single **.zip** archive and downloads it.
- Support exporting:
- A single file (still zipped, or downloaded directly if it's a single file — either is acceptable).
- A folder (recursively zipped with its structure preserved).
- Multiple selected files/folders at once (all bundled into one zip).

## 3. Additional context

- This should reuse the existing file browser UI in the project files panel — just add a selection mode and an "Export" / "Download as zip" button.
- Large exports should show progress feedback (packaging → downloading) rather than blocking silently.
- This significantly improves the workflow for users who want to take deliverables (code, documents, generated assets) out of Syncless and share them elsewhere.

Start by inspecting the existing project files view and project files panel to find how files and folders are selected and downloaded. Add an export action that packages selected files or folders into one zip while preserving folder structure and showing packaging and download progress. Done means single files, folders, and multiple selections can be exported successfully.

Written by the indexing model from the issue text.