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/chat/MessageInput.tsx

@@ -172,11 +172,9 @@ export function MessageInput({ channelId, channelName }: MessageInputProps) {
             className="p-3 text-discord-text-muted hover:text-discord-text-secondary transition-colors sticky top-0"
             title="Attach file"
           >
-            <div className="bg-discord-text-muted/20 hover:bg-discord-text-muted/40 rounded-full p-0.5 transition-colors">
-              <svg width="24" height="24" viewBox="0 0 24 24" fill="currentColor">
-                <path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm5 11h-4v4h-2v-4H7v-2h4V7h2v4h4v2z" />
-              </svg>
-            </div>
+            <svg width="24" height="24" viewBox="0 0 24 24" fill="currentColor">
+              <path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm5 11h-4v4h-2v-4H7v-2h4V7h2v4h4v2z" />
+            </svg>
           </button>
           <input
             ref={fileInputRef}