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- Archive outdated design experiments and Discord-specific assets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/server/src/routes/dm.ts

@@ -16,7 +16,7 @@ import type {
   PaginatedQuery,
   Attachment,
   Reaction,
-} from '@opencord/shared';
+} from '@backspace/shared';
 
 function sanitizeUser(row: typeof schema.users.$inferSelect): User {
   return {