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- Archive outdated design experiments and Discord-specific assets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/auth.ts

@@ -4,7 +4,7 @@ import { getDb, schema } from '../db/index.js';
 import { hashPassword, verifyPassword, signJwt } from '../utils/auth.js';
 import { generateSnowflake } from '../utils/snowflake.js';
 import { config } from '../config.js';
-import type { RegisterRequest, LoginRequest, AuthResponse, User } from '@opencord/shared';
+import type { RegisterRequest, LoginRequest, AuthResponse, User } from '@backspace/shared';
 
 function sanitizeUser(row: typeof schema.users.$inferSelect): User {
   return {