fix(app): use file URLs for app protocol assets

[janburzinski]

Description

• fixes packaged windows renderer loading
• user proper file:///C:/...
• keeps traversal guard unchanged

Checklist

• I kept this PR small and focused
• I ran a self-review before opening this PR
• I ran the relevant local checks or explained why not
• I updated docs when behavior or setup changed
• I added or updated tests when behavior changed, or explained why not
• I only added comments where the logic is not obvious
• I used Conventional Commits for commit
  messages and, when possible, the PR title

[greptile-apps]

Greptile Summary

This PR fixes how the desktop app serves packaged renderer files through the app protocol. The main changes are:

• Adds pathToFileURL for filesystem-to-URL conversion.
• Uses proper file URLs for requested app assets.
• Uses the same conversion for the index.html fallback.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The new Node built-in import is supported by the main-process runtime.
• The URL conversion improves Windows and special-character path handling without changing the traversal guard.

Important Files Changed

Filename	Overview
apps/emdash-desktop/src/main/app/protocol.ts	Replaces raw file:// string construction with pathToFileURL(...).href for app protocol asset fetches and fallback fetches.

_{Reviews (1): Last reviewed commit: "fix(app): use file URLs for app protocol..." | Re-trigger Greptile}