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- Archive outdated design experiments and Discord-specific assets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/web/src/stores/uiStore.ts

@@ -1,6 +1,6 @@
 import { create } from 'zustand';
 import { persist, createJSONStorage } from 'zustand/middleware';
-import type { User } from '@opencord/shared';
+import type { User } from '@backspace/shared';
 
 type ModalType =
   | 'createServer'
@@ -100,7 +100,7 @@ export const useUIStore = create<UIState>()(
       setPipCollapsed: (collapsed) => set({ pipCollapsed: collapsed }),
     }),
     {
-      name: 'opencord-ui-settings',
+      name: 'backspace-ui-settings',
       storage: createJSONStorage(() => localStorage),
       partialize: (state) => ({
         memberListOpen: state.memberListOpen,