fix: consistent avatar gradient colors across all 17 call sites

Avatar fallback gradients were hashed by display name alone when no
user prop was passed, causing the same person to appear in different
colors across messages, replies, member lists, voice panels, etc.

Added userId prop to Avatar and wired it through all 17 call sites
so the gradient always hashes by user ID.

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

@@ -269,6 +269,7 @@ export function PictureInPicture() {
               <Avatar
                 name={displayParticipant.username}
                 size={64}
+                userId={displayParticipant.userId}
               />
               {speakingParticipantIds.has(displayParticipant.identity) && (
                 <div className="absolute -inset-1 rounded-full ring-2 ring-status-online animate-pulse" />