Fix start with system registration

[enaboapps]

Summary

• Replace Electron login-item startup registration with deterministic HKCU Run registration.
• Verify StartupApproved state and expose non-sensitive registration diagnostics.
• Add bounded startup diagnostics JSONL logging.
• Show a small Settings warning when Windows disables or points startup at an old path.

Validation

• npm test -- src/main/system-startup.test.ts src/main/windows-startup-registry.test.ts src/main/startup-diagnostics.test.ts src/main/single-instance.test.ts
• npm test
• npm run build

Closes #249