feat: Phase 5+6 Aether Drift — legacy token removal, colorful avatars & Backspace branding

Phase 5: Remove all 54 legacy Discord token definitions from tailwind.config.js,
migrate body classes in index.html to Aether Drift tokens, update ARCHITECTURE_AUDIT.md.

Phase 6: Replace flat purple fallback avatars with deterministic colorful gradients
(mint, sky, lavender, coral, rose, teal, amber) based on user/server ID hash.
Replace Discord logo SVG with Backspace "B" in server strip. Add per-server gradient
icons with hover glow. Update profile popout banner and incoming call modal to match.

packages/web/src/components/voice/IncomingCallModal.tsx

@@ -1,6 +1,7 @@
 import React, { useEffect, useRef } from 'react';
 import { useVoiceStore } from '../../stores/voiceStore';
 import { wsSend } from '../../hooks/useWebSocket';
+import { getAvatarGradient } from '../../utils/gradients';
 
 export function IncomingCallModal() {
   const incomingCall = useVoiceStore((s) => s.incomingCall);
@@ -54,7 +55,7 @@ export function IncomingCallModal() {
         <div className="relative p-8 flex flex-col items-center gap-4">
           {/* Caller avatar */}
           <div className="relative">
-            <div className="w-20 h-20 rounded-full bg-accent-primary flex items-center justify-center text-white text-3xl font-bold">
+            <div className="w-20 h-20 rounded-full flex items-center justify-center text-white text-3xl font-bold" style={{ background: getAvatarGradient(incomingCall.callerId, incomingCall.callerName).gradient }}>
               {incomingCall.callerName.charAt(0).toUpperCase()}
             </div>
             {/* Ringing phone icon */}