Link only citations that have a source card to jump to

A pill for [N] beyond the answer's sources, or on an answer whose sources
are hidden, scrolled to nothing.

frontend/src/conversation/AnswerFlow.tsx

@@ -26,6 +26,8 @@ type AnswerFlowProps = {
   // Absent on reload, where the order is synthesized from the flat fields.
   segments?: AnswerSegment[];
   isStreaming?: boolean;
+  /** How many sources the answer can cite; see `MarkdownAnswer`. */
+  sourceCount?: number;
   agentId?: string;
   /** Set when the bubble already carries its own progress UI (a research run). */
   suppressStatusLine?: boolean;
@@ -51,6 +53,7 @@ export default function AnswerFlow({
   toolCalls,
   segments,
   isStreaming,
+  sourceCount,
   agentId,
   suppressStatusLine,
   artifacts,
@@ -136,6 +139,7 @@ export default function AnswerFlow({
             <MarkdownAnswer
               content={message}
               isStreaming={isStreaming}
+              sourceCount={sourceCount}
               artifacts={artifacts}
               turnArtifacts={turnArtifacts}
               onOpenArtifact={onOpenArtifact}

frontend/src/conversation/ConversationBubble.tsx

@@ -279,15 +279,18 @@ const ConversationBubble = forwardRef<
       </div>
     );
   } else {
+    const showSources = !(
+      DisableSourceFE ||
+      type === 'ERROR' ||
+      sources?.length === 0 ||
+      sources?.some((source) => source.link === 'None')
+    );
     bubble = (
       <div
         ref={ref}
         className={cn('flex flex-wrap self-start', className, 'group flex-col')}
       >
-        {DisableSourceFE ||
-        type === 'ERROR' ||
-        sources?.length === 0 ||
-        sources?.some((source) => source.link === 'None')
+        {!showSources
           ? null
           : sources && (
               // Stretched, not shrink-to-fit: the grid below sizes off this box,
@@ -446,6 +449,9 @@ const ConversationBubble = forwardRef<
             toolCalls={toolCalls}
             segments={segments}
             isStreaming={isStreaming}
+            // A citation pill jumps to a source card, so an answer whose
+            // sources are not shown has nothing to cite.
+            sourceCount={showSources ? (sources?.length ?? 0) : 0}
             agentId={agentId}
             // A research run already narrates itself above; the status line
             // would be a second live indicator away from the point of action.