Desktop: installed app can't reach its bundled backend (started without its own working folder) #75

Blocks #36.

What happens

In the installed Windows app, signing in fails with "Can't reach the InterviewCopilot backend. Is the desktop app running?". The bundled backend (resources\backend\InterviewCopilot.Api.exe) is running and answers /health. Started by hand from its own folder, everything works.

Cause

BackendProcess.start (src/desktop/src/backend-process.ts) spawns the backend without a working folder, so it inherits Electron's. ASP.NET uses the working folder as its content root, so it never loads the appsettings.json next to the exe. Cors:AllowedOrigins is then empty, and every request from the UI (app://interview-copilot) is rejected by CORS, which the UI reports as "can't reach". The desktop health probe isn't subject to CORS, so no error dialog appears. This has probably gone unnoticed because earlier checks had a dev backend already on port 5480, which the app reuses.

Acceptance criteria

Given the packaged app, When it starts its bundled backend, Then the backend's working folder is the folder that contains the exe
Given a fresh install with nothing on port 5480, When I sign in, Then sign-in works

Out of scope

Writing the backend's output to a log file (worth a separate issue), and any change to the backend.