refactor(chat): remove dead syntaxTheme plumbing

The Prism syntaxTheme prop is no longer read after code highlighting moved to
the Shiki worker. Remove the now-dead prop threading and its source.

- Drop syntaxTheme from interfaces, destructures, prop passes, and React.memo
  comparators across ChatMessage, MessageBody, ProgressiveGroup, ToolPart,
  TurnActivity, ToolOutputDialog, and ChatInput.
- Drop the unused _syntaxTheme param from renderWebSearchOutput.
- Remove the dead generateSyntaxTheme usages (ChatMessage memo, PlanView
  unassigned memo) and delete the now-unimported syntaxThemeGenerator module.

packages/ui/src/components/chat/ChatMessage.tsx

@@ -2,7 +2,6 @@ import React from 'react';
 import type { Message, Part } from '@opencode-ai/sdk/v2';
 import { useShallow } from 'zustand/react/shallow';
 
-import { defaultCodeDark, defaultCodeLight } from '@/lib/codeTheme';
 import { MessageFreshnessDetector } from '@/lib/messageFreshness';
 import { useConfigStore } from '@/stores/useConfigStore';
 import { useFeatureFlagsStore } from '@/stores/useFeatureFlagsStore';
@@ -12,7 +11,6 @@ import { useSessionUIStore } from '@/sync/session-ui-store';
 import { useSelectionStore } from '@/sync/selection-store';
 import { useDeviceInfo } from '@/lib/device';
 import { useThemeSystem } from '@/contexts/useThemeSystem';
-import { generateSyntaxTheme } from '@/lib/theme/syntaxThemeGenerator';
 import { cn } from '@/lib/utils';
 
 import type { AnimationHandlers, ContentChangeReason } from '@/hooks/useChatAutoFollow';
@@ -548,13 +546,6 @@ const ChatMessage: React.FC<ChatMessageProps> = ({
         return false;
     }, [themeVariant]);
 
-    const syntaxTheme = React.useMemo(() => {
-        if (currentTheme) {
-            return generateSyntaxTheme(currentTheme);
-        }
-        return isDarkTheme ? defaultCodeDark : defaultCodeLight;
-    }, [currentTheme, isDarkTheme]);
-
     const shouldAnimateMessage = React.useMemo(() => {
         if (isUser) return false;
         const freshnessDetector = MessageFreshnessDetector.getInstance();
@@ -1026,7 +1017,6 @@ const ChatMessage: React.FC<ChatMessageProps> = ({
                                                 isUser={isUser}
                                                 isMessageCompleted={isMessageCompleted}
                                                 messageFinish={messageFinish}
-                                                syntaxTheme={syntaxTheme}
                                                  isMobile={isMobile}
                                                  alwaysShowActions={alwaysShowMessageActions}
                                                  hasTouchInput={hasTouchInput}
@@ -1060,7 +1050,6 @@ const ChatMessage: React.FC<ChatMessageProps> = ({
                                                 isUser={isUser}
                                                 isMessageCompleted={isMessageCompleted}
                                                 messageFinish={messageFinish}
-                                                syntaxTheme={syntaxTheme}
                                                  isMobile={isMobile}
                                                  alwaysShowActions={alwaysShowMessageActions}
                                                  hasTouchInput={hasTouchInput}
@@ -1113,7 +1102,6 @@ const ChatMessage: React.FC<ChatMessageProps> = ({
                                 messageFinish={messageFinish}
                                 messageCompletedAt={messageCompletedAt ?? undefined}
                                 messageCreatedAt={messageCreatedAt ?? undefined}
-                                syntaxTheme={syntaxTheme}
                                  isMobile={isMobile}
                                  alwaysShowActions={alwaysShowMessageActions}
                                  hasTouchInput={hasTouchInput}
@@ -1146,7 +1134,6 @@ const ChatMessage: React.FC<ChatMessageProps> = ({
                 <ToolOutputDialog
                     popup={popupContent}
                     onOpenChange={handlePopupChange}
-                    syntaxTheme={syntaxTheme}
                     isMobile={isMobile}
                 />
             </React.Suspense>