feat: add Backspace design prototype and rebrand from Opencord

- Add Backspace-design-prototype.html: finalized "Aether Drift" design
  language with warm matte surfaces and subtle frosted glass accents
- Update CLAUDE.md with DESIGN SYSTEM section and remove Discord clone references
- Rename all Opencord references to Backspace across the full codebase
- Archive outdated design experiments and Discord-specific assets
- Add science-backed accessibility fallback (prefers-reduced-transparency)

packages/web/src/utils/permissions.ts

@@ -8,4 +8,4 @@ export {
   hasPermissionBit,
   permissionsToString,
   stringToPermissions,
-} from '@opencord/shared/src/permissions';
+} from '@backspace/shared/src/permissions';