[FEATURE]: desktop: add Open with default app action to file preview #59470

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Summary

In the desktop app's Review / file preview pane, add an "Open with default app" action for the current file, especially binary files such as .zip, .docx, .xlsx, and .pdf. The action should open the file with the operating system's default application, equivalent to double-clicking it in File Explorer/Finder.

Current behavior

• Opening a binary file in Review shows only a placeholder: file icon, path, and Binary file · <size>.
• The file tab header/toolbar shows the file name, change count, and size, but no action to open the file.
• Text/code files have an "Open in" menu (VS Code, Cursor, terminal, file explorer), but there is no "default app" action, and binary previews do not expose that menu in the top-right area at all.

Expected behavior

• Add an "Open with default app" button (or an entry in the existing "Open in" menu) in the file preview's top-right toolbar.
• It should call the existing desktop open-path IPC (for example FilesOpenPath / desktop.launch(path)) without an application argument, so the OS opens the file with its default handler.
• It should work for any file inside the session workspace and show an error/toast if the file no longer exists.

Why

Users previewing generated reports and archives (zip/docx/xlsx/pdf) often need to open them in Word, Excel, or the file manager. Today they must leave the Review pane, locate the file in the file tree or file manager, and open it there.

Steps to reproduce

1. Open a session whose workspace contains a .zip (or .docx/.xlsx) file.
2. Open the file from Review / Files.
3. The preview shows Binary file · <size> with no way to open it externally.
4. Check the top-right of the preview: there is no "Open" action.

Environment

• opencode desktop: 2.1.4
• OS: Windows 10.0.19045 (x64)
• Install/channel: release / latest
• Active plugins: [email protected]

Additional context

• The desktop bridge already exposes an open-path call used by other surfaces, so this looks like a UI exposure gap rather than a new backend capability.
• Related but different: [FEATURE]: Open local HTML previews in Chrome on macOS #57708 (open local HTML previews in Chrome), [FEATURE]: Allow users to customize how project files are opened from the TUI (e.g. Typora, VS Code, Notepad++) #48648 (TUI custom open apps).