feat(desktop): add Tauri v2 desktop app with SSE notifications

[raaymax]

Summary

• Add Tauri v2 desktop application with Rust-based SSE notification service for instant, private push notifications
• Includes system tray integration, credential storage, and multi-platform build support (Linux, macOS, Windows)
• Add frontend Tauri detection in notifications.ts to bridge IPC commands for start/stop notification service
• Rename /api/mobile route to /api/notifications with legacy alias for backwards compatibility
• Add ADR 015, desktop app docs, implementation plan, and CI workflow for cross-platform builds

Test plan

• Verify desktop/src-tauri compiles with cargo check in the desktop/src-tauri directory
• Verify Tauri dev mode works with npm run tauri dev in desktop/
• Test SSE notification connection to a running Quack server
• Verify system tray appears and window hide/show works
• Verify /api/notifications endpoint responds (same as /api/mobile)
• Test frontend Tauri detection does not break PWA notification flow
• Verify CI workflow triggers on desktop/** path changes

[github-actions]

Docker image

ghcr.io/raaymax/quack:build-90

[github-actions]

Docker image

ghcr.io/raaymax/quack:build-91

[github-actions]

Docker image

ghcr.io/raaymax/quack:build-92

[github-actions]

Docker image

ghcr.io/raaymax/quack:build-93