feat(FN-3062): add /clear command to Chat and Quick Chat, fix session banne

The merge adds a `/clear` command to both the Chat view and Quick Chat that clears transient chat state on session resets, with tests for all new hooks and components. It also persists session banner dismissals via a new `useSessionBannerPref` hook, adds a hide-banner setting, and preserves planning

Fusion-Task-Id: FN-3062

packages/dashboard/app/components/ChatView.tsx

@@ -979,18 +979,13 @@ export function ChatView({ projectId, addToast }: ChatViewProps) {
     [createSession, addToast, isMobile],
   );
 
-  // Handle send message including pending attachment uploads.
-  const handleSend = useCallback(() => {
-    const trimmed = messageInput.trim();
-    const files = pendingAttachments.map((attachment) => attachment.file);
-    if ((!trimmed && files.length === 0) || !activeSession) return;
+  const clearComposerState = useCallback(() => {
     setMessageInput("");
     setShowSkillMenu(false);
     setSkillFilter("");
     setMentionPopupVisible(false);
     setMentionFilter("");
     setMentionStartPos(-1);
-    sendMessage(trimmed, files);
     setPendingAttachments((prev) => {
       for (const attachment of prev) {
         if (attachment.previewUrl) {
@@ -999,7 +994,41 @@ export function ChatView({ projectId, addToast }: ChatViewProps) {
       }
       return [];
     });
-  }, [messageInput, pendingAttachments, activeSession, sendMessage]);
+  }, []);
+
+  // Handle send message including pending attachment uploads.
+  const handleSend = useCallback(() => {
+    const trimmed = messageInput.trim();
+    const files = pendingAttachments.map((attachment) => attachment.file);
+    if ((!trimmed && files.length === 0) || !activeSession) return;
+
+    if (trimmed === "/clear") {
+      clearComposerState();
+      stopStreaming();
+      clearPendingMessage();
+      void createSession({
+        agentId: activeSession.agentId,
+        modelProvider: activeSession.modelProvider ?? undefined,
+        modelId: activeSession.modelId ?? undefined,
+      }).catch(() => {
+        addToast("Failed to clear conversation", "error");
+      });
+      return;
+    }
+
+    clearComposerState();
+    sendMessage(trimmed, files);
+  }, [
+    messageInput,
+    pendingAttachments,
+    activeSession,
+    clearComposerState,
+    stopStreaming,
+    clearPendingMessage,
+    createSession,
+    addToast,
+    sendMessage,
+  ]);
 
   const focusComposerInput = useCallback(() => {
     if (typeof window === "undefined") return;