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- Archive outdated design experiments and Discord-specific assets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

@@ -15,7 +15,7 @@ export async function utilRoutes(app: FastifyInstance): Promise<void> {
     try {
       const response = await fetch(url, {
         headers: {
-          'User-Agent': 'OpencordBot/1.0',
+          'User-Agent': 'BackspaceBot/1.0',
         },
       });