feat: Discord Dark theme overhaul — darker palette, floating controls, UI polish

Shift entire color palette from Discord's Ash theme to the darker Dark theme.
Redesign VoiceControlBar as floating auto-show pill, add gradient Join Voice
screen, move invite icon to server header, add voice channel user status
badges, update auth pages with subtle gradient backgrounds, and sweep all
hardcoded hex colors across 14 files to match the new darker palette.

packages/web/src/components/auth/RegisterPage.tsx

@@ -45,8 +45,9 @@ export function RegisterPage() {
   };
 
   return (
-    <div className="min-h-screen flex items-center justify-center bg-discord-bg-tertiary">
-      <div className="w-full max-w-[480px] bg-discord-bg-surface rounded-md p-8 shadow-elevation-high">
+    <div className="min-h-screen flex items-center justify-center bg-[#080a0b] relative">
+      <div className="absolute inset-0 bg-[radial-gradient(ellipse_at_top,rgba(88,101,242,0.06)_0%,transparent_50%)]" />
+      <div className="w-full max-w-[480px] bg-discord-bg-surface rounded-md p-8 shadow-elevation-high relative z-10">
         <div className="text-center mb-6">
           <h1 className="text-2xl font-bold text-discord-text-primary">Create an account</h1>
         </div>