feat: unify app, skill, and composer workflows

[GCWing]

Summary

• add app-work launch tracking and skill package validation/management across the shared core, desktop APIs, and Apps/Skills/Tools surfaces
• introduce shared AppWindow, PopupMenu, FloatingCard, and Sparo logo contracts, then align workspace menus and notifications with them
• replace the legacy composer context registry with document-based context snapshots, while refining voice completion, rich-text input, and message editing flows

Validation

• cargo fmt --check --package sparo-core --package sparo-desktop
• cargo check -p sparo-desktop
• pnpm run type-check:web
• focused Vitest: 10 files, 29 tests passed

Scope note

Standalone design/QA documents and local preview/generated assets remain uncommitted.