Add desktopQuickEntryShortcut to frontend settings with a Desktop
App section in General settings, only visible when running in the
Electron app. The setting syncs to the desktop main process via
IPC whenever settings are loaded or saved.
Add a complete OAuth 2.0 PKCE flow for the Electron desktop app:
- Implement PKCE code generation and token exchange in Electron
- Register custom protocol handler (vikunja-desktop://) for deep links
- Handle deep link race conditions (buffered URLs, process.argv fallback)
- Prevent duplicate IPC listener accumulation on re-mount
- Preserve sub-paths in OAuth authorize URL for non-root deployments
- Add token refresh support using Electron's net module
