fix(web-ui): make startup welcome tabless

[bobleer]

Summary

• start the application shell with zero scene tabs and a nullable active scene
• render the existing welcome greeting as the shell-owned no-tab surface
• preserve first-scene companion opening and peer-switch reset behavior
• update focused tests and first-run documentation

Verification

• pnpm --dir src/web-ui exec vitest run src/app/stores/sceneStore.test.ts src/app/scenes/SceneViewport.test.tsx src/app/scenes/registry.test.ts src/app/components/NavPanel/components/MiniAppEntry.activity.test.tsx
• pnpm run check:web
• browser QA at 1500x894 in light mode: zero startup tabs, welcome copy visible, first Settings navigation creates real tabs, reload restores zero tabs

Remote scenarios

• Peer Device Mode: reset behavior covered by focused store test
• Remote workspace, remote control, and detached dispatch: not exercised because this change does not alter commands, protocols, persistence, or transport behavior