[P1] Improve first-run onboarding and shortcut discoverability

[nkanf-dev]

Summary

Make the first-use experience more self-explanatory and expose critical shortcuts without requiring users to discover them by accident.

Why

The interaction model is ambitious, but many high-value actions are hidden behind keyboard shortcuts or mode-specific behavior that new users are unlikely to find.

Current evidence

• File import relies on a hidden input trigger in src/app/App.tsx.
• Quick Flip and related shortcuts are described inside the overlay, after the user has already discovered it.

Scope

• Improve the empty/welcome state with clearer import affordances.
• Expose key shortcuts and gestures in a persistent or first-run help surface.
• Consider drag-and-drop import and/or a sample document entry point.
• Validate copy and flows against a new-user journey.

Acceptance criteria

• A new user can discover how to import a PDF and access quick flip without prior training.
• Shortcut help is visible before the feature is first used.
• The empty state is informative rather than decorative only.

Out of scope

• Full onboarding tour with multi-step product education.