feat(desktop): refresh the empty session workspace

[IchenDEV]

Summary

• refresh the empty-session workspace with a centered task-first hierarchy and four functional prompt starters
• quiet the sidebar, session toolbar, composer, checkout row, and plugin contribution surfaces
• keep compact windows usable with icon-density header actions and a responsive 2x2 starter layout

Validation

• bun test — 536 passed
• bun run build
• bunx tsc --noEmit
• bun run check:design — 0 new violations
• native macOS QA plus 1440x900 and 800x600 responsive visual checks
• verified that selecting a starter inserts and focuses the localized prompt