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- Archive outdated design experiments and Discord-specific assets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

@@ -7,7 +7,7 @@ import { eq } from 'drizzle-orm';
 import fs from 'fs';
 import path from 'path';
 import { pipeline } from 'stream/promises';
-import type { Attachment } from '@opencord/shared';
+import type { Attachment } from '@backspace/shared';
 
 export async function uploadRoutes(app: FastifyInstance): Promise<void> {
   // Ensure upload directory exists