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- Archive outdated design experiments and Discord-specific assets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/desktop/src/preload.ts

@@ -1,6 +1,6 @@
 import { contextBridge, ipcRenderer } from 'electron';
 
-contextBridge.exposeInMainWorld('opencord', {
+contextBridge.exposeInMainWorld('backspace', {
   platform: process.platform,
   showNotification: (title: string, body: string) => {
     ipcRenderer.send('show-notification', { title, body });