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
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