Fix: Clear chat messages when creating new project

🐛 Problem

When clicking "New Project" button, the chat messages from the previous project were not cleared, causing confusion for users.

Additionally, when switching between projects, the welcome screen briefly flashed before loading history messages, creating a poor user experience.

✅ Solution

1. Message Clearing: Added a useEffect hook that listens to route changes and clears messages/project info when:

   • Switching between projects
   • Creating a new project
   • Selecting different templates on the new project page

2. Loading State: Added isLoadingProject state to show a loading spinner instead of welcome screen while fetching project history, eliminating UI flicker.

🔧 Key Improvements

• Used location.key to catch navigation even when pathname stays the same
• Used finally block to ensure loading state is always cleared
• Only show loading state for existing projects (not for new/pending projects)

🧪 Testing

• ✅ Switching from project A to project B → messages cleared, loading spinner shown
• ✅ Clicking "New Project" from existing project → messages cleared immediately
• ✅ Selecting different template on new project page → messages cleared
• ✅ Switching from new project to existing project → messages cleared, loading spinner shown
• ✅ No welcome screen flicker when switching projects