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/voice/PictureInPicture.tsx

@@ -223,7 +223,7 @@ export function PictureInPicture() {
   return (
     <div
       ref={containerRef}
-      className={`fixed z-[40] overflow-hidden rounded-lg shadow-2xl ring-1 ring-white/10 bg-[#111214] select-none ${
+      className={`fixed z-[40] overflow-hidden rounded-lg shadow-2xl ring-1 ring-white/10 bg-[#080a0b] select-none ${
         isDragging ? 'cursor-grabbing' : 'cursor-grab'
       }`}
       style={{
@@ -249,7 +249,7 @@ export function PictureInPicture() {
           style={{ imageRendering: 'auto' }}
         />
       ) : (
-        <div className="w-full h-full flex items-center justify-center bg-[#2b2d31]">
+        <div className="w-full h-full flex items-center justify-center bg-[#1e1f22]">
           {displayParticipant ? (
             <div className="relative">
               <Avatar