feat: native application menu bar via Tauri menu API

[j4rviscmd]

Summary

Implement a native application menu bar using Tauri's native menu API, replacing the HTML-based menu bar with OS-standard look and feel.

Context

This was originally part of Phase 6 (Platform Features) but has been deferred to post-MVP. Phase 6 is now marked complete without this item.

Scope

• Application menu bar (File, Edit, View, etc.) via Tauri native menu API
• macOS: integrate with system menu bar
• Windows/Linux: standard application menu bar
• Keyboard shortcut integration

Related

• Deferred from Phase 6 roadmap in PR docs: update README screenshots and mark Phase 6 complete #217