CE: Add first-launch onboarding flow

[ericflo]

What

Add a guided setup wizard for first-time users of the ModelRelay desktop app.

Why

New users need to configure their backend URL and relay server credentials on first launch. Without onboarding, they land on an empty dashboard with no guidance.

How

• Onboarding wizard with 3 steps: Backend URL → Relay Server + API Key → Test Connection
• New Tauri command get_has_saved_settings checks if the user has configured a worker secret
• On first launch (no saved settings), the wizard overlay appears instead of the dashboard
• On successful connection test, settings are saved with auto-start enabled and the wizard dismisses
• Matches existing dark theme (bg #0f1117, cards #1a1b23, accent #6366f1)
• Step dots show progress through the wizard
• The "Test Connection" step saves settings, starts the worker, and verifies connectivity

Files changed

• crates/modelrelay-desktop/src/main.rs — added get_has_saved_settings command, registered in invoke handler
• crates/modelrelay-desktop/ui/index.html — added onboarding wizard HTML/CSS/JS