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- Archive outdated design experiments and Discord-specific assets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

@@ -323,7 +323,7 @@ export const useVoiceStore = create<VoiceState>()(
       }),
     }),
     {
-      name: 'opencord-voice-settings',
+      name: 'backspace-voice-settings',
       version: 6,
       migrate: (persistedState: any, version: number) => {
         if (version === 0) {