fix(app): persist queued followups across project switches

[nexxeln]

summary

• persist the session followup queue in workspace storage instead of keeping it only in Session component memory
• preserve queued followups when switching projects in the shared app code used by both web and desktop
• keep existing queue send behavior unchanged once the user returns to the session

testing

• bun test src/utils/persist.test.ts
• bun typecheck in packages/app currently fails due to an unrelated existing remend module/types error in packages/ui/src/components/markdown-stream.ts