fix(web): Add dismissible blur backdrop to popups

[itswcl]

Summary

• Add a shared popup backdrop with blur and click-to-dismiss behavior.
• Apply it to alerts, theme/account dropdowns, group management popovers, group sheets, and ticker group assignment menus.
• Prevent backdrop clicks from reaching dashboard cards or other page controls behind open popup surfaces.

Verification

• cd apps/web && npm run lint
• cd apps/web && npm run build
• git diff --check