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/message/parts/ProgressiveGroup.tsx

@@ -35,7 +35,6 @@ interface ProgressiveGroupProps {
     isExpanded: boolean;
     collapsedPreviewCount?: number;
     onToggle: () => void;
-    syntaxTheme: Record<string, React.CSSProperties>;
     isMobile: boolean;
     expandedTools: Set<string>;
     onToggleTool: (toolId: string) => void;
@@ -373,7 +372,6 @@ type AggregatedRow =
 interface ExpandableToolRowProps {
     activity: TurnActivityPart;
     isExpanded: boolean;
-    syntaxTheme: Record<string, React.CSSProperties>;
     isMobile: boolean;
     onToggleTool: (toolId: string) => void;
     onShowPopup: (content: ToolPopupContent) => void;
@@ -385,7 +383,6 @@ interface ExpandableToolRowProps {
 const ExpandableToolRow: React.FC<ExpandableToolRowProps> = ({
     activity,
     isExpanded,
-    syntaxTheme,
     isMobile,
     onToggleTool,
     onShowPopup,
@@ -402,7 +399,6 @@ const ExpandableToolRow: React.FC<ExpandableToolRowProps> = ({
             part={activity.part as ToolPartType}
             isExpanded={isExpanded}
             onToggle={handleToggle}
-            syntaxTheme={syntaxTheme}
             isMobile={isMobile}
             onContentChange={onContentChange}
             onShowPopup={onShowPopup}
@@ -425,7 +421,6 @@ const ExpandableToolRow: React.FC<ExpandableToolRowProps> = ({
 
 const MemoExpandableToolRow = React.memo(ExpandableToolRow, (prev, next) => {
     return prev.isExpanded === next.isExpanded
-        && prev.syntaxTheme === next.syntaxTheme
         && prev.isMobile === next.isMobile
         && prev.onToggleTool === next.onToggleTool
         && prev.onShowPopup === next.onShowPopup
@@ -826,7 +821,6 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
     isExpanded,
     collapsedPreviewCount = 0,
     onToggle,
-    syntaxTheme,
     isMobile,
     expandedTools,
     onToggleTool,
@@ -915,7 +909,6 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
                         key={row.activity.id}
                         activity={row.activity}
                         isExpanded={expandedTools.has(row.activity.id)}
-                        syntaxTheme={syntaxTheme}
                         isMobile={isMobile}
                         onToggleTool={onToggleTool}
                         onShowPopup={onShowPopup}
@@ -942,7 +935,6 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
                         key={row.activity.id}
                         activity={row.activity}
                         isExpanded={expandedTools.has(row.activity.id)}
-                        syntaxTheme={syntaxTheme}
                         isMobile={isMobile}
                         onToggleTool={onToggleTool}
                         onShowPopup={onShowPopup}