GitHub OAuth callback opens Headlamp instead of AKS Desktop

## Problem

When a user completes the "Connect GitHub" OAuth flow and has both Headlamp and AKS Desktop installed, the OAuth callback opens Headlamp instead of AKS Desktop.

## Root Cause

The deep link protocol scheme was hardcoded as `headlamp://` in the Electron main process. Both AKS Desktop and Headlamp registered the same `headlamp://` protocol, so the OS could route the `headlamp://oauth/callback` redirect to the wrong app.

## Fix

- The protocol scheme is now read dynamically from `package.json`'s `build.protocols.schemes` at startup, so each Headlamp-based app uses its own registered scheme.
- AKS Desktop's scheme is set to `aks-desktop://` in its `package.json`.
- The GitHub OAuth App's callback URL must be updated to `aks-desktop://oauth/callback`.

## Required Configuration Change

The GitHub OAuth App (client ID `Iv23liWWbvrfIrA6WWj5`) needs its **Authorization callback URL** updated to `aks-desktop://oauth/callback` in the GitHub App settings.

Review the Electron main-process startup path and package.json build.protocols.schemes entry; confirm the AKS Desktop value is aks-desktop://. Then inspect the GitHub OAuth App settings and verify its Authorization callback URL is aks-desktop://oauth/callback; done means the Connect GitHub flow opens AKS Desktop when both apps are installed.

Written by the indexing model from the issue text.