fix(security): harden Electron MCP navigation boundaries

[tatoalo]

Problem

our electron renderer trusted packaged file: navigation using an encoded pathname prefix and only observed main-frame navigation. Renderer subframes could therefore reach unsupported external protocols without passing through the validated host link launcher, while the persistent session retained Electron's default openExternal permission behavior.

Changes

• allow main-frame navigation only to the exact renderer entry file
• observe subframe navigation and block schemes that can leave the browser context, while preserving supported MCP sandbox and embedded web content
• deny renderer-originated openExternal permission checks and requests for the persistent session
• isolate packaged Electron tests with temporary user-data directories and add a real subframe-navigation regression test

[trunk-io]

😎 Merged manually by @tatoalo - details.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit e2b5aae.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(test): create Electron E2E user data..." | Re-trigger Greptile}

[github-actions]

All changes are strictly additive security hardening — tighter file-URL matching, subframe navigation allowlist, and session-level openExternal blocking. The resolved CodeQL alert was a false positive on a hardcoded test constant.