Manifest editor exposes arbitrary local file existence through checkImagePath

Describe the bug

The manifest editor extension host handles untrusted webview checkImagePath messages by calling fs.existsSync on absolute paths and paths resolved outside the manifest/workspace roots. Its imagePathStatus response reveals whether an arbitrary local path exists.

This was surfaced while resolving #71. The file-copy sink is now separately gated by an opaque token and native user confirmation; this issue is limited to the remaining existence oracle and is intentionally out of scope for #71.

To Reproduce

1. Open an AppxManifest file in the custom manifest editor.
2. From a compromised or malicious webview script, post repeated checkImagePath messages containing absolute local paths.
3. Observe imagePathStatus: external for existing paths and notFound otherwise.

Expected behavior

Webview-originated path checks should be restricted to trusted package/workspace roots. External paths should only be inspected after explicit native user selection or another extension-host-controlled authorization step.

Screenshots

N/A

OS Version and details

Windows 11

Start at the manifest editor extension-host handler for checkImagePath and trace how paths become the imagePathStatus response. Verify the reproduction with absolute paths, then restrict webview-originated checks to trusted package or workspace roots; external paths should require explicit native selection or extension-host authorization.