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- Archive outdated design experiments and Discord-specific assets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)

.env.example

@@ -1,4 +1,4 @@
-# Opencord Configuration
+# Backspace Configuration
 # Copy this file to .env and fill in the values
 
 # Server
@@ -15,7 +15,7 @@ LIVEKIT_API_SECRET=
 
 # Storage
 UPLOAD_DIR=./data/uploads
-DB_PATH=./data/opencord.db
+DB_PATH=./data/backspace.db
 MAX_UPLOAD_SIZE=104857600
 
 # Registration — set to false to disable new user registration