fix(ui): harden mobile bottom overlays

[audemed44]

Summary

• centralize the mobile bottom-nav offset into shared layout tokens
• keep bulk selection actions above the mobile nav and reserve page space while selecting
• convert edit modals to a stable scroll shell with sticky action footers on mobile

Testing

• npx tsc --noEmit
• npx eslint src --ext js,jsx,ts,tsx --fix
• npm test